Harrowing had no effect on oyster spat settlement. Where live spat were recorded in 2014, three times as many were settled in unharrowed than harrowed plots. Indeed, the majority of spat were recorded in Plot 2, the only plot which remained fallow for the 3 years. This supports a previous finding that harrowed areas did not receive as much settlement as fallowed areas (Waugh, 1972). The results support the view that where a fishery is reliant on natural spatfall, there is little that harrowing can do to influence settlement (Spencer, 2002). Recruitment in Lough Foyle is highly subject to natural variations in reproductive cycles (McKelvey et al., 1996; Andrew, 2002), and in 2012 and 2013, there was poor settlement throughout the lough (Bromley, 2015). Other than Anomia ephippium, settlement by other bivalves was equally low and unpredictable with, for example, Pecten maximus only recorded in 2012 and 2013 but not in 2014. Anomia ephippium would not represent a good proxy for oyster settlement. This species was recorded in comparably high abundances in both management treatments and in years with poor O. edulis recruitment. Nor did it appear to be selective of substratum, settling on clean and heavily fouled shell and non-shell substrata.
The numbers of spat were very low, and it is difficult to draw firm conclusions with regard to substratum preferences. However, together with observations from other work in the lough (Bromley, 2015), that all but one oyster had settled on oyster shell lends some support to the view that O. edulis preferentially settles on live oysters or oyster shell (Cole & Knight-Jones, 1939). However, the amount of fouling appeared to be less important, with only one oyster settled on clean shell.
Harrowing did have some effect on substratum composition. The weight of shell and the percentage of non-oyster shell had decreased in the harrowed plots by 2014. Clean, darker shell had also become more common in the harrowed plots. This type of shell has been buried for some years and is regarded as useful in the Blackwater because O. edulis spat are believed to seek out dark-coloured settlement surfaces (Cole & Knight-Jones, 1939; Walne, 1964; Haward & Bird, 2012, pers. comm.). Blackened shell was discarded in the 1972 study of harrowing because it had been buried, though no reason is given for this (Waugh, 1972). Unfortunately, given the low spatfall, we were unable to prove or disprove this hypothesis. Shell bored by Cliona celata was less common in harrowed plots. As such shell is fragile, this indicates that harrowing may have caused it to disintegrate. The results indicate that harrowing is analogous to the reported effects of dredging in terms of homogenising substrata and breaking up shell (Kaiser et al., 2000; Brown et al., 2010; Tully & Clarke, 2012). It has been observed that whilst dredging can be beneficial to some beds, too much of it can turn shell to sand and reduce the size of the oyster beds (Holt, 1903). Similarly to bivalve settlement, significant differences in the amount of fouling by organisms such as Pomatoceros triqueter and barnacles can be attributed to inter-annual cycles in setting intensity, rather than the effects of harrowing (Knight-Jones & Stevenson, 1950; Knight-Jones, 1953).
There were suggestions in the data that harrowing could have a negative effect on the community associated with oyster beds. Harrowing reduced the number of species contributing to both similarities and dissimilarities between plots. Reduced species richness and higher abundance of predators and scavengers such as Buccinum undatum, Carcinus maenas and Pagurus bernhardus are indicators of disturbance (Collie et al., 1997; Murawski et al., 2000). This may impact on fishery and conservation management objective under European directives (Tully & Clarke, 2012). In addition to Features of Conservation Interest (FOCI) species such as O. edulis and Arctica islandica, the Annex I species/habitat eelgrass (Zostera marina) is present in the lough and studies have shown that harrowing or raking can damage eelgrass beds (Fonseca et al., 1984; De Jonge & De Jonge, 1992; Everett et al., 1995; Tallis et al., 2009).
References to harrowing and long-standing use of the technique in some oyster production areas (Fowler, 1893; Laing et al., 2005; Haward, 2012, pers. comm.) may suggest to managers that harrowing should be adopted to recondition oyster beds in their location. However, managers need question whether it is indeed necessary and whether, for example, cultch addition may be a more effective use of resources.
Cole’s (1956) manual for oyster cultivation is the source of many recent references, suggesting the use of harrowing for habitat remediation. However, this manual for oyster cultivation did not consider harrowing to be a “one size fits all” method. The main context within which it was recommended was (i) for removing silt accumulated on neglected grounds and (ii) preparing cultivated areas for laying spat or halfware brought in from elsewhere for on-growing (analogous to a farmer ploughing a field ready for planting seed) (Cole, 1956). Indeed, the author suggested that reconditioning barren or neglected grounds may not work, work can be expensive, and may damage stock or spread disease (Cole, 1956). Habitat restoration work may need to be carried out for many years and still have no effect (Korringa, 1951).
The only previous published study of the effects of harrowing concluded that harrowing did not increase settlement, and had long-term adverse effects on the growth and survival of existing stock (Waugh, 1972). It was recommended that any such work should be “carefully considered and weighed up against potential long-term interference” (Waugh, 1972). More recently, the oyster bailiffs in the Fal have not only harrowed where they have identified a problem, for example, after an outbreak of bonamiosis, left one bed derelict but also found that the technique had no effect in reconditioning the bed (Ferris, 2012).
In an active fishery, harrowing may be redundant. Harrowing was advocated for the Lough Foyle beds 20 years ago owing to oysters being buried under piles of cultch (Cunningham, 1991). However, a later study found little evidence of this, suggesting that increased fishing activity has redistributed cultch (McKelvey, 1996). It has been stated that there is no need or only occasional need to carry out harrowing as a “maintenance measure” where there is regular fishing (Webster & Merritt, 2011; Woolmer et al., 2011). It was also concluded during the experiments in the Rivers Crouch and Fal that dredge harvesting was more effective than harrowing at preparing oyster grounds for spat collection (Waugh, 1972). Although not experimentally tested, it was apparent in Lough Foyle in 2014 that Perch bed, one of the most heavily fished beds, attracted much higher spatfall than Redcastle (pers. obs. and unpublished Loughs Agency data).
One of the main tasks for which harrowing is recommended is removal of silt (Cole, 1956; Webster & Merritt, 2011). The one scenario where harrowing may indeed be necessary in fisheries which are located in depositing systems in estuarine salt marsh creeks and muddy substrata, where silt build-up can smother newly laid stock and inhibit spat settlement (Dean, 1893; Fowler, 1893; Knight-Jones, 1953; Hancock, 1955; Webster & Merritt, 2011). This, together with the control of the invasive Crepidula fornicata, is the explanation for annual harrowing being carried out in the Blackwater (Hancock, 1955; Haward, 2012, pers. comm.). The Lough Foyle results indicate that silt build-up in the experimental area is unlikely to be the cause of lack of oyster settlement. Although at certain times, the water column in the lough can carry high sediment loads, tidal currents in Redcastle are sufficiently strong to remove sediment. The fact that only the 2014 samples were significantly different from the controls and there was no significant difference between sampling stations suggests that 2014 sampling coincided with a period of high turbidity. Other than the small amounts of the blue clay noted during pre-experiment camera work, there was no mud in the dredge samples, especially compared to the amount of mud which is present in dredge hauls on mussel beds in Lough Foyle or in the Blackwater (pers. obs.).
Dislodging and disturbing sediment may also have negative effects. It has been suggested that estuarine sediments may act as an important winter food reservoir for oysters and other shellfish in the Foyle (McKelvey, 1996). Also, in locations with a long history of shipping traffic, such as Lough Foyle, heavy metals and the now banned antifouling agent tributyltin (TBT), associated with imposex in molluscs, may be sequestered in the sediment and could be released back into the water column (Arakawa et al., 1971).
In areas such as the experimental site in this study where habitat enhancement efforts have proved ineffective, other causes need to be investigated to identify the best regeneration strategy (Fowler, 1893; Cole, 1956). In addition to the influence of inter-annual variations in reproductive cycles, it would be reasonable to suggest that with such a low stock density (0.037 oysters/m2), reproductive output would be low due to the Allee model (Allee et al., 1949). This also means that, if the presence of conspecifics is an important driver of settlement substratum selection in oysters, there are too few oysters left on the bed to attract spat (Cole & Knight-Jones, 1939). The strong currents (>1.5 knots) that can occur in the area could assist in removing silt but may also inhibit settlement and transport larvae away from the bed (Woolmer et al., 2011).
Also, previously, productive grounds can cease to support oysters owing to changes in abiotic conditions. This may be indicated by benthic community composition. Abundant starfish were considered to be an indicator of suitable oyster ground, together with ascidians, whelks, hermit crabs and slipper limpets (Cole, 1956). Absence can thus be as informative as presence. For example, the common starfish, Asterias rubens, can be a major oyster predator but can be excluded by fluctuating or low salinities (Hancock, 1955, 1969; Mackenzie, 1970). In this study, ca. 50% of the shell examined was fouled with either all or a proportion of dead Pomatoceros triqueter and barnacle spp. Dead fouling organisms on shell either show that the shell has been buried (which was interpreted in this study as another indication that harrowing had exposed buried shell) or indicates that abiotic conditions have changed and are less suitable for oysters than they were in the past (Cole, 1956; Burke et al., 2008).
In the absence of dredge fishing, the remains of spat (valves and attachment marks) found in all 3 years indicates losses through natural mortality. Present in dredge samples were a number of potential oyster spat predators—Ocenebra erinacea, Nucella lapillus, Cancer pagurus, Carcinus maenas and Buccinum undatum (Mackenzie, 1970; Smyth & Roberts, 2010). In common with the O. edulis spat, Ocenebra erinacea was more common in unharrowed plots. This disagreed with the observation that the favoured conditions for spatfall (clean shell with little silt) are also favoured by oyster drills (Cole, 1956).
Although there was no modern experience of harrowing in Lough Foyle, the methods for the experiment were based on advice from the published literature (Cole, 1956; Waugh, 1972) and Blackwater oystermen (Haward & Bird, 2012, pers. comm.). It was confirmed that the dredge was working on the seabed and the 2012 pilot was used to refine the methods for 2013 and 2014. There are some aspects which could have influenced the results. A skeleton dredge may not be the most efficient harrow type. Although these have been viewed as “particularly useful for disturbing cultch at the beginning of the breeding season” (Cole, 1956), in the USA, agricultural harrows are perceived to be better at maintaining habitat than bagless dredges, partly because the latter are narrower and cover less ground (Webster & Merritt, 2011). This was addressed by having small areas (2 ha each) to be harrowed and ensuring that the harrowing was carried out to ensure all the plot was covered repeatedly. Debris was also regularly removed from the dredge to prevent this impinging on its action on the seabed (Cole, 1956). Timing should not have influenced the results. The 2012 pilot was carried out at the end of June, after the oysters had started spawning, and harrowing in 2013 and 2014 was carried out at the end of May to ensure that the work was completed before spawning commenced. It has generally been recommended for harrowing to start in June but also that this needs to take into account local conditions, for example, starting at the end of May in the Blackwater so that beds are ready for spatfall at the end of June (Cole, 1956). Having addressed as many potential confounding factors as possible, and, in view of poor settlement throughout the lough in 2012 and 2014 and the few significant differences in the results, we believe these considerations to be insufficient to cause any change to the conclusions.
The recommendation of this study would be that harrowing should never be carried out on productive oyster beds which are already subject to intense fishing activity. Neither should harrowing ever be carried out once native oysters have commenced spawning as this carries the risk of damaging or disturbing newly settled spat. Whether harrowing may be applied to reconditioning long-neglected beds should be assessed on an individual basis for each proposed site. Potential effects on the benthic faunal and floral assemble should be taken into account, especially where conservation protections are applicable.