Untangling the effects of fishing effort and environmental variables on benthic communities of commercially fished scallop grounds

The Isle of Man fishing industry is currently predominated by two lucrative and heavily exploited scallop fisheries, targeting Pecten maximus and Aequipecten opercularis. The impacts of which have previously been investigated, however without the addition of environmental information. This thesis represents a unique long-term investigation into the impacts of fishing pressure and environmental variables on the benthic invertebrate communities of fishing grounds found around the Isle of Man. A significant positive trend in seawater temperature was found, along with an inverse correlation with chlorophyll-α. Fishing pressure was found to have a small, significant negative effect on indices of diversity; however environmental variables were unable to explain the remaining patterns in diversity. The composition of each of the benthic communities was then investigated in more detail. Fishing pressure had a significant negative effect on densities of benthic invertebrates at some grounds; however this study showed that many of the heavily fished sites were composed of dredge-tolerant species. Significant relationships were found between the densities of Asterias rubens and Porania pulvillus and several environmental variables on the south-west fishing grounds, suggesting that environmental variation, rather than fishing pressure was responsible for variations in these species. Further evidence was found of the negative impact of scallop dredging from the long-term analysis of a closed area, implemented in 1989. Recovery of P. maximus has occurred within this closure, without the concurrent increase in of the predatory starfish A.rubens. Relationships between several benthic species with the closed area and environmental variables were found. However, the results of this study indicate a complex ecosystem, which is also affected by predator-prey interactions. The overall findings of this research indicate that closed area management is a relatively straightforward and effective measure in this region. Future management decisions will however, have to account for the potential effects of climate change.