Improved protocol for collecting mussel watch specimens taking into account sex, size, condition, shell shape, and chronological age

Abstract

This study was done to determine the relative effects of five variables (sex, soft tissue dry weight, condition index, width∶height ratio, chronological age) on the concentrations of 24 elements in the blue musselMytilus edulis and to develop an improved protocol for collecting mussels for biological monitoring programs. The five explanatory variables were treated as independent variables in a multiple regression equation with the individual element concentrations being included as the dependent variable. A multivariate test was also performed. An initial test showed that chronological ageper se had no significant effect on the concentrations of any of the elements; therefore, it was dropped from the equation. Condition index and soft tissue dry weight showed a high degree of negative association with element concentrations. This was explained as being due to growth rate differences (dilution effect). An improved condition index is suggested. Sex was also a major factor in determining element concentrations with the greatest effects being noted for manganese, copper, arsenic, and selenium (females greater than males). The width∶height ratio showed a lesser but significant positive effect on element concentrations which was also considered to be due to growth rate differences (dilution effect). It is recommended that mussels only be collected from sites with similar maximum shell lengths. To ensure similar growth rates, all mussels from all sites should be of similar shell length and approximately 70–90% of the maximum potential length observed at the particular site where collected. Preexamination of mussels at each site should be done to ensure that the average condition index and width:height ratio (shell shape) of the mussels to be collected are similar. It is preferable to collect mussels subtidally, if available, during the late winter before significant spawning activity. Separate composite samples of male versus female mussels should be analyzed. When the above protocol is not feasible, an alternate statistical method is suggested which will enhance the interpretability of the results.