Lethal and non-lethal effects of trawling on the benthic invertebrate food web

Abstract

The pulse trawl is presented as an alternative for the traditional tickler chain beam trawl. The beam trawl has a direct lethal effect on the benthic community, which is much less for the pulse trawl. However, the pulse trawl, due to its electrical field has non-lethal effects on the benthic community. The existence of such non-lethal effects have been demonstrated for different organisms and most often involve a short period of inactivity. Such a period of inactivity may have consequences at the population level as it may limit feeding. Yet the population dynamical consequences, nor food web level consequences, have been studied in experimental or field settings. To study non-lethal and lethal effects of trawling on the benthic community a model was developed which is based on a trait-based characterization of the food web. This model describes the equilibrium dynamics of predators, filter feeders and deposit feeders. The model includes direct mortality as a function of fishing mortality and a non-lethal effect, which was incorporated as a decrease of the maximum food intake rate. This approach was based on experiments with electric pulse and benthic invertebrates. We conducted a comparative analysis of lethal and non-lethal impact of trawling on the equilibrium dynamics of the benthic food web. Only lethal effects represent a tickler chain beam trawl and a combination of lethal and non-lethal effect represents a pulse trawl. We first studied the effects of lethal and non-lethal impacts on the food web separately. We then studied the trade-off between lethal and non-lethal effects for a given total impact on the community. This approach shows how much non-lethal impact is required to compensate for a decrease in lethal effects of trawling. The study shows that lethal effects need to be replaced by non-lethal effects with similar strength in order to maintain the effect of fishing intensity on the food web. Such strong non-lethal effects are unlikely to occur in the field based on experiments assessing the impact of electricity on benthic macro-invertebrates and trawling intensity. In other words, the combined non-lethal and lethal effects of the pulse trawl are unlikely to have a similar effect on the macro-invertebrate benthic community compared to the lethal effects of the traditional trawl.