Predicting benthic fauna biomass in the Voordelta under different hypothetical fisheries regimes : Could an expected increase in benthic fauna have occurred within an area protected from demersal trawling if shrimp fisheries should not have increased?

Abstract

The aim of this study is to evaluate if an increase in benthic biomass and longer-lived species in the Voordelta area could have been possible if the shrimp fishery would not have increased over the study period. At the instigation of the bottom protection area, there were no signs yet of a very active shrimp fishing fleet, let alone that shrimp fisheries would expand substantially in magnitude. For this reason, the current study evaluates the ‘what-if’ specific changes in the fishery had not happened, how would benthic biomass have developed over time. The study uses the PMR benthic monitoring data and assigns longevity classes to each of the species found in the monitoring data. Cummulative longevity distributions are calculated and a statistical framework is used to estimate how these distributions are affected by fishing, wind and substrate. The resulting statistical model estimates are used in a population model in which we can simulate biomass development per longevity class under a number of fishing scenarios. Five scenarios were evaluated and shown in the figure below, demonstrating that restricting fishing activity by any kind of fleet would have resulted in maximum 5% change in biomass in any particular year. Even if fishing effort would have continued at similar levels as the 2004-2005 baseline situation, including a closure of the bottom protection area in 2008, there would have been a mimimal increase in biomass in the Voordelta area. As demersal fishing has declined more rapidly in recent years than expected (in comparison with the 2004-2005 baseline), there is no marked difference between the expected current state of benthos biomass in the Voordelta and the 04-05 baseline scenario.