Growth models for assessing anthropogenic impacts on King scallop, Pecten maximus (Bivalvia), at Frøya, Norway

In marine ecosystems, the performance of coastal species is affected by anthropogenic activities such as aquaculture and plastic pollution. These impacts will increase in the future as the human population and consumption grow. It is crucial to obtain the true effects of environmental factors on ecosystems and their organisms for management and scientific purposes. Hence, precise growth models requiring easily recorded data and with biologically relevant and interpretable parameters are needed. King scallop Pecten maximus is a commercially harvested filter feeding bivalve species populating large parts of coastal Europe. Here, I used King scallop as a model species to assess the effect of these anthropogenic stressors in an outer coastal region in Trøndelag, Norway. Traditionally, growth of this species has been modelled by the Von Bertalanffy growth equation, known to model early age classes imprecisely, reducing accuracy for assessing environmental impacts. I used the least square criterion to compare this growth model to an alternative model based on the Gompertz growth equation, with equally many parameters. A total of n = 89 specimens were sampled from n = 3 locations, one of which had been exposed to aquaculture since 2014. Results demonstrates the Gompertz growth equation models the growth of this species more accurately than the Von Bertalanffy, producing less outliers, having lower residual variance and less heteroscedasticity. This demonstrates that the G is more suitable for growth analysis than the VB. Generalised linear models and mixed effect models were applied to determine if presence of aquaculture or birth year influences growth by model selection using AIC. Model results show a strong effect of birth year, while only maximum growth rate from the Gompertz function and the maximum shell height from the Von Bertalanffy show significant effects of aquaculture. Parameter estimates from the two growth models produce ambiguous results, highlighting that selecting the most correct growth function is essential to avoid erroneous conclusions. Nevertheless, based on the Gompertz results, aquaculture seems to influence growth performance, although small sample size and few sampling sites strongly advocates cautious interpretation. The apparent effect of birth year could reflect temperature increases in the water masses, large-scale and perhaps delayed effects of aquaculture, or a selection pressure due to increased harvest. Lastly, an attempt to assess effects of plastic content in scallops failed as the method for analysing plastic content proved unsuitable, i.e. hydrogen peroxide did not adequately digest the biogenic material. A series of methodological adjustments suggest using other digestive agents known from literature such as HNO3 or KOH, together with Nile Red dye for increased visibility and NaCl for density separation in this method of rapid screening of plastic content.