Introduction

Small-scale fisheries (SSFs) play a major socioeconomic role within the Mediterranean fishing sector, accounting for 82% of the total fishing fleet (FAO 2023). These fisheries typically involve small vessels (LOA < 12 m) and operate either full- or part-time throughout the year, employing a diverse array of fishing strategies and gears in coastal areas and targeting multiple species (Grati et al. 2018; Calò et al. 2022; Papageorgiou and Moutopoulos 2023). Passive nets, particularly gillnets, are the most common gear used in Mediterranean SSFs (Lucchetti et al. 2020).

In the North-Western Adriatic Sea (FAO Geographical Sub-Area 17), one such fishery is operated by bottom-set gillnets, mainly targeting mantis shrimp (Squilla mantis) and common sole (Solea solea), but they also frequently capture and land other species due to their commercial value and additional income (Fabi and Grati 2005). However, this fishery is no longer profitable in the region for several reasons. Spatial conflicts, i.e. when several fishing activities operate simultaneously on the same fishing grounds (Bergh et al. 2023), are common in the overexploited areas of the Adriatic Sea (Farella et al. 2021; Grati et al. 2022; Tassetti et al. 2023), and can have a negative impact. Furthermore, the dolphin depredation on gillnets is a matter of concern in the area (Pardalou et al. 2022). It is caused primarily by the common bottlenose dolphin (Tursiops truncatus), which ‘steals’ fish from the net, resulting in catch loss, damage to the gear, and accidental entanglements of dolphins. Overexploited fish stocks may have increased the frequency of depredation (Reeves et al. 2013). Additionally, the economic crisis experienced by SSFs may have exacerbated fishers’ perceptions of dolphins as competitors (Li Veli et al. 2023).

Gillnets often exhibit poor species selectivity, which, in a multispecies fishery, can result in considerable bycatch levels and consequent discards (Tsagarakis et al. 2014, 2017). Discards for bottom gillnets in the Adriatic Sea are generally around 20% of the total catch in weight, with a potential increase in some seasons (Fabi and Grati 2005; Lucchetti et al. 2020). Moreover, gillnets in the area are responsible for relevant bycatch rates of marine megafauna, such as marine mammals (Đuras et al. 2021), sharks (Di Lorenzo et al. 2022), and turtles (Casale 2011; Lucchetti 2021; Virgili et al. 2024a).

Given these premises, it is crucial to guarantee socio-economic sustainability for SSFs i.e. landings, profits, and incomes, while ensuring sustainable exploitation of resources. Recently, the European Commission funded various scientific initiatives to investigate and promote pots as potential alternatives to traditionally used gear (Petetta et al. 2021), while also addressing the perspectives of stakeholders.

Pots can have a higher potential to reduce bycatch in some fisheries when compared to passive set nets (Petetta et al. 2021; Suuronen et al. 2012). Moreover, the removal of any discards from pots upon gear retrieval has considerably improved the survival probability compared to gillnets, as the trapped animals are kept alive in the water during the pot soaking time (i.e., the time between setting and hauling when gears are fishing; Barragán-Méndez et al. 2020; Lorenzon et al. 2013). For the same reason, several pot fisheries provide higher quality catches compared to other gear types (Olsen 2014). Additionally, pots are less susceptible to depredation (e.g., by dolphins and seals; Königson 2011; Pusch 2011), as access to the catch is limited for large-bodied animals, especially in the small pots employed in the Mediterranean basin (Petetta et al. 2020). Nevertheless, according to the last available data (year 2021), the weight and value of landings generated by Mediterranean pot fisheries account for approximately 3,200 tons and € 24 million, respectively, representing 1% of total fishery landings and 1.6% of their revenues (STECF 2023). Therefore, Mediterranean fishers can be encouraged to use pots as a method of capturing certain target species, at least in some areas and seasons. This approach would diversify the fishing fleet and mitigate the impact on non-target species (Petetta et al. 2021).

Mantis shrimp is one of the top-target demersal species in terms of landings for the Italian national fleet and, specifically for the SSFs, ranking among the top 6 species in 2021 and representing around 10% (in weight) of gillnet landings (STECF 2023). The aim of this study was to assess whether pots can be used in small-scale mantis shrimp fisheries to replace gillnets. Although mantis shrimp pots have been used since the second half of the last century, their expansion in the past was restricted due to competition with active gears targeting the same species (Froglia 1996). However, in recent years, these pots have increasingly been used along the entire Western Adriatic coast, especially in the summer and early autumn, when trawling in coastal areas is forbidden. Therefore, this study was aimed at answering the following questions:

  • Does species composition differ between gillnet and pot catches in a small-scale coastal fishery targeting mantis shrimp?

  • Is there any difference in catch efficiency between pots and traditional gillnets in this fishery?

  • Does the economic value of the catch differ between gillnets and pots, and could pots be used in this fishery as an alternative gear without negatively affecting the profit of this SSF?

Materials and Methods

Experimental design and sea trials

The study used a conventional bottom-set gillnet (hereinafter GNS, following FAO fishing gear catalogue; ISSCFG 2016) as a control. The GNS was composed of multiple netting panels made of transparent polyamide monofilament (diameter 0.20 mm) joined in the same gang. Each panel was 50 m in length and 3.6 m in stretched net drop (50 meshes in height), although the real vertical opening during fishing was relatively low (< 1 m), due to the low floatability of the float line. The mesh size was 72 mm. The float line and lead line were polypropylene (diameter 5.5 and 4.5 mm, respectively); the lead line weighed 60 g/m. The horizontal hanging ratio, i.e. the slack of the netting panel, was 0.33 for the float line and lead line, which are common values in Mediterranean gillnets (Lucchetti et al. 2015). The tested GNS was the same in all fishing operations.

The pots targeting S. mantis were used as a treatment (hereinafter FPO, ISSCFG 2016), and they presented similar characteristics to those commercially used along Western Adriatic SSFs. Each FPO had a semi-ellipsoidal shape with dimensions of 32 cm length, 30 cm width and 14 cm height. It had a volume of 12.1 L and a weight of 0.55 kg. The FPO frame (diameter 2 mm; rectangular mesh size, 10 mm length and 22 mm width) was made of galvanized metal with green plastic coating to give more resistance to oxidation. The frame of the entrance was oval-shaped (internal surface: 18.8 cm2) and positioned in the front side. The entrance funnel was 12 cm long and made in plastic of the same colour of the FPO. On the opposite side of the entrance, sideways, there was a box, made of plastic, which served as bait container. The FPO structure was made so that the opening placed on the upper side is regulated by means of an internal elastic rope; this mechanism allowed ease baiting and emptying the pot with a quick operation of lifting and closing. Fresh sardines were used as bait: 3 individuals per pot for a total of about 40 gr. FPO were attached to a propylene main line (diameter 8 mm) by 1.5 m branch lines (diameter 5 mm) with a quick release swivel. This allowed to ensure a correct setup depending on the force and direction of bottom water current. Considering that FPO once lowered the first time were no longer brought ashore until the end of the fishing season (except for some washing and descaling operation) the use of anchors (Danforth©, United Arab Emirates) was required to face periods of strong wave motion and bad seawater conditions. FPO were placed at 15 m intervals according to the traditional rigging used by the local fishers (Petetta et al. 2020; Virgili et al. 2024b).

Each fishing trial utilized a range of 500 to 750 m of GNS (corresponding to 10–15 panels) and a gang of 40 FPO (Fig. 1) to obtain equal total length for each gear. The GNS and FPO deployments were paired, as each deployment consisted of setting and retrieving GNS and FPO gangs on the same fishing grounds and were spaced around 0.25 nautical miles apart (Fig. 1). The soaking time varied from half a day to almost a whole day for GNS and from a few hours to days for FPO.

Fig. 1
figure 1

TOP: Technical design and specifications of the two gears tested: conventional gillnet (GNS) used in Adriatic SSFs to target S. mantis and other demersal species, and pot for S. mantis (FPO). BOTTOM: Illustration of the sampling method: commercial vessel (LOA 6.8 m; gross tonnage 1 GT; gasoline-powered engine 100 HP) with a crew of two fishers, equipped with a net winch to haul 500–750 m of GNS and 40 FPO

Full size image

A typical commercial vessel with a crew of two fishers was used for sea trials. Activities were conducted during the late summer or autumn of two consecutive years (September–November 2021 and September–October 2022), when the maximum yield of the target species (Grati et al. 2018) and dolphin depredation on set nets (Li Veli et al. 2023) were expected. Fishing operations were conducted in shallow waters (depth < 15 m) at 1–3 nautical miles off the Pedaso coast (Western Adriatic Sea, Italy), where fishing grounds are characterized by sandy bottoms inhabited by S. mantis.

After retrieving the GNS and FPO, the crew proceeded to sort the catch on board separately for each gear type. The total catch for each gear was thus divided into: a landed catch consisting of the main target species and other species that have commercial value; discards consisting of species with no commercial value such as invertebrates and fish species and commercial individuals under the minimum landing size or in poor condition; and, bycatch of protected, endangered, and threatened species (PET species included in EU regulations and international lists such as the EU Habitat Directive and the IUCN Red List), which were discarded if dead or released alive. Observers on board identified all individuals to the lowest taxonomic level possible, counted them, weighed them to the lowest 1 g on a small balance scale with marine compensation, and measured them to the nearest 0.5 cm for total length (TL; fish) or mantle length (ML; cephalopods), and to the nearest 0.1 mm for carapace length by means of a calliper (CL; S. mantis and other crustaceans). No subsampling was conducted in this study. The economic value of the commercial species caught by each fishing gear was daily recorded. The value was determined by the local market price and expressed as €/kg.

Preliminary data analysis

A preliminary data check and analysis were performed to ensure that the data could be treated as paired, as the deployments were paired between the two gears. Initially, the deployments of both GNS and FPO with soaking times that significantly deviated from the average were excluded from further analyses. A linear model (lm; R-package) (Pinheiro et al. 2018) was then applied to assess the potential effect of soak time on the mantis shrimp catch for each gear. In case no significant effect was detected, data was treated as paired.

Comparing the species composition between gillnets and pots

We performed a catch dominance analysis to quantify and compare the species composition observed in the catches of GNS and FPO, respectively (Herrmann et al. 2022; Petetta et al. 2023). This methodology enables an evaluation of the impact of fishing gear from a species-community perspective, measuring how much each species caught dominates in the sample (Warwick et al. 2008). The catch dominance analysis was performed to evaluate the relative abundance of each species in both the total catch, the discarded catch, and the landed catch.

The species dominance for GNS and FPO was estimated for each of the gear types separately, and the estimation of the species dominance pattern was averaged over deployments of pots and gillnets, respectively. Following the procedure described in (Petetta et al. 2023), a fixed rank was assigned to each single species caught in the sea trials by including it in one of the following four categories: 1) ‘Target species’, i.e. the main commercial species targeted by GNS and FPO; 2) ‘Other species of commercial value’, i.e. additional landed species with a commercial value; 3) ‘Other species of no commercial value’, i.e. those species usually discarded by fishers; 4) ‘PET species’.

The estimation of the catch dominance in the number of individuals (dni), weight (dwi), and economic value (dvi) was carried out separately for FPO and GNS and for each fraction of the catch (total, discarded, and landed) (Cerbule et al. 2022; Herrmann et al. 2022; Madhu et al. 2023; Petetta et al. 2023):

$$\begin{array}{c}{dnt}_{i}=\sum_{j=1}^{m}\left\{\frac{{nt}_{ij}}{\sum_{i=1}^{Q}\left\{{nt}_{ij}\right\}}\right\}\\ \begin{array}{c}{dwt}_{i}=\sum_{j=1}^{m}\left\{\frac{{{\rho }_{ij}\times nt}_{ij}}{\sum_{i=1}^{Q}\left\{{{\rho }_{ij}\times nt}_{ij}\right\}}\right\}\\ {dvt}_{i}=\sum_{j=1}^{m}\left\{\frac{{{\sigma }_{ij}\times nt}_{ij}}{\sum_{i=1}^{Q}\left\{{{\sigma }_{ij}\times nt}_{ij}\right\}}\right\}\end{array}\end{array}$$
(1)

where j represents the deployment and i is the species rank defined previously. nij is the count number for each species i in deployment j. Parameter ρij is the average weight of species i in deployment j in a given fraction of the catch; it is obtained from the total weight and number of individuals. Parameter σij is the average value of species i in deployment j in a given fraction of the catch. Q is the total number of species observed, and m is the total number of paired deployments considered in the analysis.

Furthermore, the cumulative dominance curves were estimated in the number of individuals (DntI), weight (DwtI), and economic value (DvtI) to represent the cumulative proportional abundances of the species (Cerbule et al. 2022; Herrmann et al. 2022; Madhu et al. 2023; Petetta et al. 2023):

$$\begin{array}{c}{Dnt}_{I}=\sum_{j=1}^{m}\left\{\frac{\sum_{i=1}^{I}{nt}_{ij}}{\sum_{i=1}^{Q}{nt}_{ij}}\right\}\text{ with }1 \le I \le Q\\ {Dwt}_{I}=\sum_{j=1}^{m}\left\{\frac{\sum_{i=1}^{I}{\rho }_{ij}\times {nt}_{ij}}{\sum_{i=1}^{S}\left\{{\rho }_{ij}\times {nt}_{ij}\right\}}\right\}\text{ with }1 \le I \le Q\\ {Dvt}_{I}=\sum_{j=1}^{m}\left\{\frac{\sum_{i=1}^{I}{\sigma }_{ij}\times {nt}_{ij}}{\sum_{i=1}^{S}\left\{{\sigma }_{ij}\times {nt}_{ij}\right\}}\right\}\text{ with }1 \le I \le Q\end{array}$$
(2)

In Eq. (2), I is the species index summed up in the nominator. The fixed species ranking in all catches in the cumulative dominance curves allows the comparison of the steepness of the cumulative dominance curves. Specifically, steeper sections in dominance curves will imply that some species are more dominant than other species in the fishing gear (GNS or FPO, respectively), while the horizontal parts show that the particular species are not abundant (Cerbule et al. 2022; Herrmann et al. 2022; Petetta et al. 2023).

The Efron percentile 95% Confidence Intervals (CIs; Efron 1982) were used to provide the uncertainty of the values of dominance patterns obtained following the procedure described in (Herrmann et al. 2022). This procedure enables the estimation of uncertainties around the dominance values at the species level induced by the limited sample sizes at a single deployment as well as the between-deployment variation in species dominance.

Furthermore, the difference ∆d in species dominance d between FPO (p) and GNS (g) was estimated for the number of individuals, weight, and economic value (Herrmann et al. 2022; Petetta et al. 2023):

$$\Delta d={d}_{p}-{d}_{g}$$
(3)

The CIs for Eq. 3 were obtained based on separate bootstrap populations for dp and dg, as described in (Cerbule et al. 2022; Herrmann et al. 2022). The significance was detected by inspecting the CIs for the difference; if the 0.0 value was within the CIs, no significant difference was detected.

Comparing the mantis shrimp catch efficiency between gillnets and pots

We estimated the relative length-dependent mantis shrimp catch efficiency of GNS and FPO to quantify the effect of gear changes on catch efficiency. We used the method described in Herrmann et al. (2017) to compare the catch data for the two gears. This method models the length-dependent catch comparison rate (CCl) summed over pot and gillnet deployments by:

$${CC}_{cl}=\frac{\sum_{j=1}^{m}\left\{{np}_{cl,j}\right\}}{\sum_{j=1}^{m}\left\{{np}_{cl,j}+\frac{{ng}_{cl,j}}{{qg}_{j}}\right\}}$$
(4)

where npcl,j and ngcl,j are the numbers of mantis shrimp caught in each size class cl for the pots and GNS, respectively, in deployment j. qgj is a sampling factor that accounts for the fact that some of the gillnet deployments used less than 750 m of net. For example, if deployment j used mlj m of gillnets, qgj is mlj/750. Parameter m is the number of paired deployments considered. We estimated the catch comparison rate CC(cl, v) expressed by Eq. (4) by minimizing the Expression (5) (maximum likelihood estimation):

$$-\sum_{cl}\left\{\sum_{j=1}^{m}\left\{{np}_{cl,j}\times ln\left(CC\left(cl,{\varvec{v}}\right)\right)+\frac{{ng}_{cl,j}}{{qg}_{j}}\times ln\left(1.0-CC\left(cl,{\varvec{v}}\right)\right)\right\}\right\}$$
(5)

where the outer summation is over the length classes cl and the inner summation is over the deployments m. Parameter v describes the catch comparison curve defined by CC(cl, v). The experimental CCcl was modelled by:

$$CC\left(cl,{\varvec{v}}\right)=\frac{exp\left(f\left(cl,{v}_{0},\dots ,{v}_{k}\right)\right)}{1+exp\left(f\left(cl,{v}_{0},\dots ,{v}_{k}\right)\right)}$$
(6)

In Eq. (6), f is a polynomial of order k with coefficients v0 to vk. We considered f up to an order of 4 with parameters v0, v1, v2, v3, and v4; leaving out one or more of the parameters v0…v4 led to 31 additional models for the catch comparison function CC(cl, v).

Among these models, the estimations of the catch comparison rate were made by using multi-model inference to obtain a combined model (Burnham and Anderson 2002; Herrmann et al. 2017). We evaluated the ability of the combined model to describe the experimental data based on the p-value, calculated by the ratio between the model deviance and the degrees of freedom (DOF; Herrmann et al. 2017; Wileman et al. 1996). A p-value > 0.05 implies that the combined model describes the experimental data sufficiently well. In cases of poor fit statistics (p-value < 0.05 and deviance/DOF > 1), the residuals were inspected to determine whether the results were due to structural problems when modelling the experimental data or data over-dispersion (Wileman et al. 1996).

With similar catch efficiency between gears, the catch comparison rate would be 0.5; thus, the application of this baseline allows us to judge whether there is a difference in catch efficiency between FPO and GNS. Since the results of CC(l,v) do not provide a direct relative value of the catch efficiency between gears, we further estimated the catch ratio CR(l,v) (Herrmann et al. 2017) as follows:

$$CR\left(l,{\varvec{v}}\right)=\frac{CC\left(l,{\varvec{v}}\right)}{\left(1-CC\left(l,{\varvec{v}}\right)\right)}$$
(7)

When the catch efficiency between FPO and GNS is similar, the value of CR(cl, v) would be equal to 1.0. If the CR(cl, v) is significantly larger than 1.0, it would show that FPO are catching significantly more mantis shrimp with cl compared to GNS.

The CR(cl, v) values provided by Eq. (7) are specific to the number of FPO versus meters of GNS used in the experiment, based on 40 FPO versus 750 m of GNS, which corresponds to 5.33 FPO per 100 m of GNS deployed. However, a typical vessel with two crew members would realistically deploy 3,000 m of GNS or 270 FPO (real scenario), which corresponds to 9 FPO per 100 m of GNS deployed. Therefore, the results provided by Eq. (7) can be scaled by a factor of 1.69 (≈ 9/5.33) to obtain a realistic comparison. For completeness, we additionally scaled the results obtained by Eq. (7) to provide estimates for 1 to 10 FPO per 100 m of GNS deployed.

The 95% CIs for CC(cl, v) and CR(cl, v) were estimated using a double bootstrapping method (Herrmann et al. 2017). The bootstrapping method accounts for between-deployment variability as well as within-deployment variability. However, contrary to the double bootstrapping method (Herrmann et al. 2017), the outer bootstrapping loop used in the current study (accounting for the variability between deployments) was conducted in pairs to take full advantage of the experimental design of deploying GNS and FPO simultaneously. By using multi-model inference in each bootstrap iteration, the method also accounted for the uncertainty in model selection. We performed 1,000 bootstrap repetitions and calculated the Efron 95% CIs.

Furthermore, length frequency distribution and cumulative length frequency distribution analyses were used to quantify the proportion of the total catch of mantis shrimp for each carapace length class cl and up to a given carapace length class CL, captured with gear type i.e. GNS (t = g) or FPO (t = p). The analysis was conducted using the following equation (Cerbule et al. 2021):

$$\begin{array}{c}{fnt}_{cl}=\frac{\sum_{j=1}^{m}{nt}_{cl,j}}{\sum_{j=1}^{m}\sum_{cl}{nt}_{cl,j}}\\ {Fnt}_{CL}=\frac{\sum_{j=1}^{m}\sum_{cl=0}^{CL}{nt}_{cl,j}}{\sum_{j=1}^{m}\sum_{cl}{nt}_{cl,j}}\end{array}$$
(8)

By incorporating the evaluation of Eq. (8) in the double bootstrap described above, we obtained 95% CIs. Further, to compare the length distribution of mantis shrimp captured by both gears, the differences in length frequency \(\Delta {fn}_{cl}\) and cumulative length frequency \(\Delta {Fn}_{CL}\) between GNS (g) and FPO (p) were estimated as follows:

$$\begin{array}{c}{\Delta fn}_{cl}={fnp}_{cl}-{fng}_{cl}\\ {\Delta Fn}_{CL}={Fnp}_{CL}-{Fng}_{CL}\end{array}$$
(9)

The Efron 95% CIs for the \({\Delta fn}_{cl}\) and \({\Delta Fn}_{CL}\) were obtained using the double bootstrapping method, as described above. All the analyses described in Sects. "Preliminary data analysis" and "Comparing the species composition between gillnets and pots" were conducted using the SELNET software (Herrmann et al. 2012, 2016, 2017, 2022).

Comparing the profits

An economic assessment was performed to determine the feasibility and economic sustainability of implementing the alternative gear in comparison to the traditional one.

To assess the profits at the gear level for each fishing operation, the main costs related to fuel consumption, staff, working time, and other additional costs, such as gear maintenance and bait, were collected. Cost items varied according to the gear used and were related to fuel consumption, additional staff (personnel for cleaning the GNS and personnel dedicated to logistics for FPO transport at sea), and other factors (e.g., maintenance of the gear and bait for FPO). The daily gasoline costs per liter, available online at the Italian Ministry of the Environment, were multiplied by the liters consumed. The additional staff costs were calculated based on the hourly cost communicated by the boat owner and the required hours of work. Finally, the profits were estimated by subtracting costs from the revenues obtained from the direct sale of fish.

The European Regulation (EU 2019) establishes that the maximum length for GNS is to be set at 6 km, whereas there is no legislation in force for pot fisheries. The length of commercially used GNS depends on the capabilities and dimensions of the fishing vessel and the strategy and attitude of the fishers. Therefore, for a crew consisting of two members, we considered the real scenario outlined in Sect. "Comparing the mantis shrimp catch efficiency between gillnets and pots" involving the deployment of 3 km of GNS and 270 FPO, i.e., the average amount of nets and pots employed by vessels similar to the one used in the study. We investigated whether there was any significance when comparing the profit obtained with the two gears, first considering profits solely from S. mantis and then from all the catch sold.

A non-parametric bootstrap method with replacement was utilized to undertake statistical comparisons of profit data between GNS and FPO (Efron and Tibshirani 1993). The variability of the profit difference means (derived from catches), considering both S. mantis alone and all the species caught, was evaluated using the non-parametric bootstrap with replacement in the following manner. From the profit data, 2,000 replicates of the same dimension (30 values) with replacement were bootstrapped for each gear. From these replicates, 2,000 values of the mean difference between GNS and FPO were derived. On this set of 2,000 differences, the 95% Efron CIs of the true difference were then calculated according to the improved BCA percentile method (Efron and Hastie 2022). If the 95% CIs did not capture the zero value, the difference was deemed statistically significant. All the calculations were carried out using the free statistical software R (R Core Team 2023). The BCA interval was calculated with the function bca of the R package “coxed” (Kropko and Jeffrey 2019).

Results

Preliminary data analysis

A total of 30 valid fishing operations were conducted in the two-year period (Table 1). The preliminary analysis excluded GNS and FPO deployments with extreme soak times. The average range of GNS soak times was 11–17 h, in accordance with common fishers’ practices. Therefore, we excluded two GNS hauls that had a soak time of 22 h, along with the corresponding FPO deployments. The average range of FPO soak time was 14–48 h; thus, five FPO hauls with soak times of 1, 72, 74, 78, and 95 h, respectively (and the corresponding GNS hauls), were excluded. Consequently, 23 out of 30 paired deployments were considered for further catch analyses.

Table 1 An overview of the catches obtained from the deployment of gillnets (GNS) and pots (FPO). The black horizontal line divides the deployments of two fishing seasons (2021, 2022)
Full size table

The lm results revealed that, within the 23 paired deployments, soak time had no effect on the mantis shrimp catch, either for GNS (estimate = 2.76; p-value = 0.39) or for FPO (estimate = -0.06; p-value = 0.98).

Comparing the species composition between gillnets and pots

GNS and FPO captured 40 different species during the fishing operations (Fig. 2, top). Only 3 species were considered as “Target species” i.e., mantis shrimp for both gears and two sole species (Solea solea and Pegusa impar) for GNS. The majority of species (25) belonged to the “Other species with commercial value” category and were mostly fish, except for the octopus (Octopus vulgaris), caramote prawn (Penaeus kerathurus), and changeable nassa (Nassarius mutabilis). The “Other Species without commercial value” category included 9 species: 5 mollusc species, 2 crab species, and 2 fish species. The “PET species” category only included the elasmobranch species caught by the GNS, i.e., the blue shark (Prionace glauca, 1 individual), sandbar shark (Carcharhinus plumbeus, 1 individual), and pelagic stingray (Pteroplatytrigon violacea, 1 individual). Additional results on catches are reported in Supplementary Material 1.

Fig. 2
figure 2

TOP: Assigned ranking (S) of the species caught, divided by category: target species, other species with commercial value (€), other species without commercial value (no €), and Protected, Endangered, and Threatened species (PETs). BOTTOM: Cumulative species dominance in the catch of pots (FPO: a, d, g) and gillnets (GNS: b, e, h). The curves (solid lines) with 95% CIs (dashed lines) represent the cumulative species dominance for catches in the number of individuals (I), weight (W), and economic value (E). Delta plots (c, f, i) resulting from the comparison of single species dominance in the total catch between FPO and GNS in the number of individuals (I), weight (W), and economic value (E). The curves (solid lines) with 95% CIs (dashed lines) represent the comparison between gears at the single species level. The grey horizontal line at 0 represents an equal proportion between the two gears. The green, yellow, orange, and red areas represent target species, other species with commercial value, species without commercial value, and PET species, respectively

Full size image

The cumulative curves of the total catch show that, in FPO, the species with commercial value (species IDs S1-S28) account for an average of 83% (CIs 77–87%) in terms of the number of individuals and 92% (CIs 89–94%) in terms of the weight of all the species caught (Fig. 2 bottom, a, d). These percentages are significantly lower in GNS i.e. 33% (CIs 23–44%) in terms of the number of individuals and 74% (CIs 64–82%) in terms of weight. Therefore, for this gear, the catch was primarily dominated by species that did not have a commercial value (Fig. 2 bottom, b, e, and Table S1).

The catch composition in FPO was dominated by S. mantis, which accounted for more than 80% and 90% of the total and landed catch in weight, respectively, with a small contribution from other species in the catch composition. S. mantis almost saturates the cumulative curve of the landed fraction in both the number of individuals and weight; 100% is reached at species 21 (black goby, Gobius niger, around 5% in weight on average; Fig. 2 bottom, a, d). In terms of weight, none of the other species with a commercial value reached an average dominance of 3% in the total catch (Table S1). Regarding the discarded fraction, the highest proportion was observed for the purple dye murex (Bolinus brandaris; around 41% in number and 49% in weight), followed by changeable nassa (Nassarius mutabilis; around 47% in number and 22% in weight; Table S1). In the latter case, despite being a commercial species with a dedicated pot fishery in the area (Grati et al. 2010), the entire catch was discarded due to a lack of demand in the autumn season.

The proportion of S. mantis in the catch was significantly less dominant in GNS than in FPO, reaching only 19% and 26% of the total and landed catch in weight, respectively (Fig. 2 bottom, e). The same average percentages were observed for common sole (S. solea). Among the other commercial species, high proportions in the catches were observed for tub gurnard (Chelidonychthys lucernus) and Mediterranean starry ray (Raja asterias), with an average of 12% and 16% of the total and landed catch in weight, respectively (Table S1). The discarded portion was dominated by two species (B. brandaris and swimming crab, Liocarcinus spp.), each representing, on average, 27% of the proportion in weight. High percentages in the discarded fraction are also observed for PET species due to the substantial weight of the few individuals caught (Fig. 2 bottom, e and Table S1).

The same trend between gears was observed when considering the economic value, since S. mantis represented more than 99% (CIs 98–100%) of the total catch in FPO (Fig. 2 bottom, g), versus 29% (CIs 19–78%) in GNS. The species with the highest economic value in GNS was S. solea (45%; CIs 11–63%), with significant contributions from other commercial species such as C. lucernus, R. asterias, and flathead grey mullet (Mugil cephalus; Fig. 2 bottom, h, and Table S1).

Delta plots between the total catch dominance curves of the two gears provide a detailed insight at each single species level (Fig. 2 bottom, c, f, i). The plots showed a marked dominance of S. mantis, picarel (Spicara maena), N. mutabilis, and conger eel (Conger conger) in the FPO catches compared to GNS in the number of individuals, weight, and economic value. By contrast, the proportion of several commercial and discarded species (species IDs S2 to S6; S8; S12; S30; S31) was significantly larger in the GNS total catch.

Comparing the mantis shrimp catch efficiency between gillnets and pots

Table 1 reports the number of S. mantis individuals measured in each deployment in both gears. The fit statistics of the combined model used in the catch comparison by pooling the deployments of the two years (p-value = 0.06; Deviance = 48.15; DOF = 34) indicated the model’s good fit to the experimental data.

Compared to GNS, FPO were significantly more efficient at capturing mantis shrimp for most length classes, except for the 29–30 mm CL classes, where the increase in catch efficiency when using FPO was not statistically significant (Fig. 3a, b). On average, FPO caught 1.5 to 3 times more mantis shrimp than GNS when considering the most abundant length classes (22 to 32 mm; Fig. 3b). The FPO catch efficiency was even higher than GNS in a real scenario, with FPO catching, on average, 2.5 to 5 times more mantis shrimp for the same length classes (Fig. 3c). The catch ratio estimates in mean curves (Fig. 4), considering 1 to 10 FPO per 100 m of GNS deployed, highlighted that 4 FPO are sufficient to guarantee a higher mantis shrimp catch than 100 m of GNS. Additional results are reported in Supplementary Material 1.

Fig. 3
figure 3

Results of the catch comparison analyses obtained for mantis shrimp between pots (FPO) and gillnets (GNS). a) modelled catch comparison rate (black line) with 95% CIs (black dashed curves); the black circles represent the experimental rate; the grey horizontal line at 0.5 represents the point at which both gears have equal catch rates. b) the catch ratio (black line) with 95% CI (black dashed curves); the blue lines represent the length frequency distributions obtained with the two gears (light blue, GNS; dark blue, pots); the grey horizontal line at 1.0 represents the point at which both gears have equal catch rates. c) catch ratio curve with CIs scaled by a factor of 1.69 (≈ 9/5.33) to obtain a realistic comparison

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Fig. 4
figure 4

Catch ratio curves attained by scaling the results obtained by the catch ratio equation provide estimates for 1 to 10 pots (FPO) per 100 m of gillnets (GNS) deployed (numbers in italics). The red curve represents the real scenario, i.e., 9 FPO per 100 m of GNS deployed

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Comparing the profits

The cost items and incomes (in mean values) for the two fishing gears are reported in Table 2. The major FPO costs were represented by fuel and bait (43% and 46%, respectively). For GNS, the major costs were fuel and additional staff (53% and 32%, respectively).

Table 2 Mean values (with standard errors) of cost items and incomes expressed in euros for the two fishing gears (GNS: gillnets; FPO: pots). The text reports values as percentages (Sect. "Comparing the profits"). Fuel: cost for fuel consumption during gear transport and fishing activities; Staff: cost for additional staff other than crew members (i.e., for cleaning the set nets); Other: costs for gear maintenance and bait for pots; TOT: total costs; MTS Inc.: income obtained when only taking into account the sale of S. mantis; APS Inc.: income obtained when taking into account the sale of all commercial species
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On average, the mean price per kilo for S. mantis was € 12.7 and € 18.0 for GNS and FPO, respectively. Figure 5 illustrates the profit comparison between the two gears. When only considering the profit obtained from S. mantis catches, we observed significantly higher profits for FPO (mean profit: € 253.4) than GNS (mean profit: € 19.2). The 95% CIs for the mean profit difference calculated according to the non-parametric bootstrap were CIs € [-174.3, -297.5]. When considering all the species caught, the mean FPO profit (€ 256.5) was slightly higher than the mean GNS profit (€ 192.8); however, the bootstrap 95% CIs for the mean profit difference were CIs € [-32.3, -146.7], highlighting that this difference was not significant.

Fig. 5
figure 5

The profit comparison between gillnets (GNS, ○) and pots (FPO,□) is detailed. The profits shown only consider S. mantis income and those profits from income obtained from the sale of all the commercial species caught

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Discussion

The purpose of this study was to test the catch efficiency and profitability of pots targeting mantis shrimp in the Adriatic Sea and comparing them with traditionally used gillnets. Specifically, the study evaluated whether pots could provide a more sustainable alternative to traditional gillnets, examining their ability to mitigate current issues in these fisheries, such as reducing bycatch and discards, and enhancing profitability.

The findings from the catch dominance and catch comparison analyses demonstrate that pots targeting S. mantis are highly selective for the target species, resulting in few discards and no PET species bycatch. This has been observed in earlier studies for other pot fisheries targeting cephalopods and shrimps (Castriota et al. 2004; Fabi and Grati 2005), and fish (Petetta et al. 2020). In contrast, the lower species selectivity of gillnets produced more discards of species with no commercial value (e.g., crabs), raising conservation concerns due their potential to capture primarily elasmobranchs and sea turtles (Lucchetti et al. 2017). High discard rates are already reported in several fisheries employing set nets, such as gillnets targeting hake (Merluccius merluccius) in the Ionian Sea (29.5%; Tzanatos et al. 2007), trammel-nets targeting prawns in Izmir Bay (Gökçe and Metin 2007), and spiny lobster (Palinurus elephas) in Tunisia and Spain (Quetglas et al. 2004), where discards may exceed 40%. In terms of bycatch, set nets in the Mediterranean Sea are responsible for the annual capture of approximately 31,000 sea turtles (mostly C. caretta) with a mortality rate of 30–50% (Lucchetti 2021; Virgili et al. 2024a, b). Much less is known about elasmobranchs and their interactions with SSFs (Di Lorenzo et al. 2022), although in the Northern Adriatic, gillnets called "cagnolare" are specifically employed to catch demersal sharks (e.g., Squalus spp., Mustelus spp.) (Bargione et al. 2019; Lucchetti et al. 2023). Low bycatch and mortality rates are observed for coastal cetacean species, particularly T. truncatus (i.e., 0.14 animals per year per vessel in Italian waters; Li Veli et al. 2023), although a strong interaction with set nets is known (Lauriano et al. 2009). Another difference resides in the discarding practices observed. While the unwanted organisms caught by pots were usually alive and promptly discarded at sea, increasing their chances of survival, the bycatch individuals in gillnets were dying or already dead when the net was retrieved. Typically, these individuals were not immediately discarded due to the additional time needed to free them from the meshes. These organisms were always removed once hauled onto the deck, without any possibility of survival. However, there are strategies in set nets that can reduce discards by increasing mesh sizes (Karakulak and Erk 2008; Lucchetti et al. 2020), reducing netting twines (Ayaz et al. 2011; Grati et al. 2015), adjusting hanging ratios (Kumova et al. 2015), and using the “guarding net” (i.e. a strip of monofilament net with large meshes placed in the lower part of trammel nets (Martínez-Baños and Maynou 2018; Sartor et al. 2018).

Moreover, pots demonstrated a significantly higher catch efficiency and catch quality for mantis shrimp compared to gillnets. This could explain the increased use of pots along the Western Adriatic coasts in recent years (Virgili et al. 2024b). It is also clear that, apart from S. mantis, the two gears attract different species, despite fishing on the same grounds. Commercial fish like common sole, tub gurnard, starry ray, grey mullet, and most sparids do not enter the small mantis shrimp pots. However, they constitute an important catch fraction of gillnets. In contrast, pots are often approached by smaller fish with (e.g., black goby) or without (e.g., blotched picarel and conger eel) economic value, which, due to their size, are not caught by the gillnet meshes. For the same reason, pots retain a higher proportion of smaller mantis shrimp than gillnets, and these are potentially marketable since the species lacks a minimum landing size (EU 2019). However, their capture may raise concern regarding the sustainability of pots for this species, and regulations or recommendations for the establishment of either a minimum landing size, minimum pot mesh size, or good practices on the release of smaller live specimens may be needed.

Pot fisheries can represent a valid strategy at the economic level, providing lower costs for personnel, fuel consumption, and other operating costs. Revenues are, in fact, comparable to those of traditionally used gillnets if the whole catch is considered, and even higher if the mantis shrimp alone is considered. In this study, GNS had a cost of around € 65 per panel, while FPO had a cost of € 9 per unit. In addition, some fishers produce pots by hand, recycling materials and components used in other fishing gear or those intended for other uses, further reducing costs. In contrast, fishing with gillnets requires an additional daily trip to set the nets, resulting in additional fuel costs. From the profit assessment, we can estimate that the initial pot costs could be amortized over the course of a quarter of the fishing season (typically of 3–4 months with an average of 10–15 fishing days per month). Of course, mantis shrimp pots require bait to effectively attract the prey, contrary to other pots employed in the area (e.g., for cuttlefish Sepia officinalis, and fish) (Petetta et al. 2020). However, the cost of sardines commonly used as bait is usually less than € 1.50 per kilo. Also, the space required on deck for pots is not an issue in this fishery since these pots are usually deployed in several trips at the beginning of the season and then left at sea throughout the season, with the movement of single gangs each time. The economic analysis includes the cited costs, demonstrating their lack of impact on FPO profits compared to GNS.

The conflict with other commercial or recreational fishers, increased fuel costs, and other fishing-related factors (e.g., dolphin depredation) disadvantage GNS more than FPO. These findings align with results coming from the “PRIZEFISH project” (2019), which highlighted a good trend in certain economic indicators such as net profit margin and return on investment (i.e., Return on Fixed Tangible Assets, RoFTA) and a general good performance of the mantis pot fisheries sector in GSA 17 (Supplementary Material 2). Pot fisheries are one of the few cases where the percentage of income from a single species is higher than 50%, compared to other passive gears that base their profit on multiple species. Price is the main factor that influences customers’ buying decisions, followed by the freshness of the product. As a result of the freshness and quality of the product, price derived from pots, as outlined in our paper, is almost double the average price obtained from other gears (i.e., trawling), and higher than that of set nets (PRIZEFISH, 2019).

The use of pots as an alternative to gillnets has shown promising results at the socioeconomic level. At least in certain seasons, this could provide small-scale fishers, who earn less from fishing activities compared to large-scale fishers (STECF 2023), with an opportunity to increase their income from landings and improve overall sustainability.

The narrow entrance of mantis shrimp pots limits access to the catch, reducing the risk of depredation by large predators such as bottlenose dolphins, which do not include this species in their diet, as observed in Mioković et al. 1999, and Neri et al. 2023.

Increased pot fishing efforts may result in the overfishing of target species and the ongoing capture of marine animals when pots are lost at sea (so-called ghost fishing). The most recent results from SAC-WGSAD (2022) indicated that the Adriatic stock is sustainably exploited. However, it is recommended that: (1) fishing mortality is not to increase; and (2) a valid stock assessment for S. mantis is carried out in this area by improving data collection on this species (biological parameters, catches and landings, effort, and prices).

Ghost fishing is a critical problem in some Mediterranean fisheries. Ghost fishing mostly involves gillnets (Ayaz et al. 2006), longlines, and trawls (Ayaz et al. 2010). However, there is a dearth of research specifically focusing on lost pots (Petetta et al. 2021). Generally, pots allow non-target species to escape relatively easily (Ungfors et al. 2013), and their fishing potential has been found to be low after bait consumption (Adey et al. 2008). In our study, the pots were securely rigged, preventing any loss due to adverse weather conditions and underwater currents.

Recently, there has been an increase in the use of S. mantis pots and other pots targeting fish and cuttlefish (Petetta et al. 2021; Virgili et al. 2024b) along the Italian coasts in terms of the number of boats involved (PRIZEFISH, 2019). This expansion is likely linked to the exclusion of active gear, especially from coastal areas, as established by current legislation. In fact, EU Regulation (2006) prohibits trawling within three nautical miles, while the Italian Ministerial DM 399/2019 extends this prohibition to six nautical miles for specific periods (following the summer fishing ban). Moreover, the use of AIS (Automatic Identification System) has become mandatory for vessels over 15 m in LOA (Length Overall), ensuring greater control over fishing activities, particularly in certain coastal areas. The resulting reduction in gear conflicts with active gear has decreased the effective fishing effort, benefiting the SSF fleet (Bastardie et al. 2017) in terms of exploitable fishing grounds and fishing days. Therefore, diversification of the fleet on a spatio-temporal scale could be a solution, especially for the multispecies and multigear fisheries in coastal areas. Providing fishers with an ecolabeled fish product to access more lucrative markets is also a forward-looking strategy for those who voluntarily use low-impact fishing gear (Virgili et al. 2024a).

Conclusion

The results of this study show that pots could replace gillnets and trammel nets following the seasonality of several SSF target species. A proposed solution is to replace the use of set nets with a year-round fishing method that involves the use of different types of pots. Specifically, pots would be used to catch cuttlefish from March to July, mantis shrimp from July to November, basket traps for changeable nassa from November to March, and occasionally, foldable fish pots for seabreams (i.e. Diplodus spp.), red mullet (Mullus barbatus), and octopus (Petetta et al. 2021). The diversification of the fleet ensures a lower fishing effort on a broader spectrum of demersal species and guarantees greater competitiveness on the fish market. The use of pots to replace traditionally used GNS in this S. mantis fishery constitutes an endeavor to achieve greater sustainability in terms of both economic viability and environmental impacts. To further improve sustainability in this pot fishery, it may be necessary to implement additional management measures, particularly by imposing regulations on the maximum number of pots to be deployed per vessel, setting a minimum mesh size for the pots, and defining spatio-temporal closures.