Loss of food resource to predators and Dietary Equivalence Assessment
Fish Entrapment in Power Station Cooling Water Systems
catches can be converted to Equivalent Adult Values (EAVs). The Equivalent Adult Values concept standardises the biological value of fish of different ages by considering species specific reproductive and mortality rates to estimate the number of larval and juvenile fish that would have survived to reach adulthood had they not been entrapped. Converting Equivalent Adult Values to a biomass then allows the weight of adult equivalents entrapped to be expressed in the context of other anthropogenic pressures which target adult fish specifically, such as commercial fishing activities. Whilst historically this has been the most common use for Equivalent Adult Values, Jacobs has recently been developing this methodology to enable the quantitative assessment of an indirect effect of fish entrapment - the loss of food resource to predators.
indirect effect to higher trophic levels if, for example, the fish predicted to be entrapped were removed from an area considered to be a high intensity feeding ground for predators. In this scenario, displacement of predators and possible effects to population vital rates (e.g. mortality and reproduction) could occur, and further investigation into the resilience of feeding strategies to meet physiological requirements would be required. By integrating Equivalent Adult Values into what Jacobs has termed a Dietary Equivalence Assessment (DEA), larval and juvenile fish which have been entrapped within power station cooling water systems can be quantified in terms of their future potential value as a dietary resource to predators such as sea birds and marine mammals. Figure 1. outlines the relationship between Equivalent Adult Values and Dietary Equivalence Assessments in more detail. Very simply, a Dietary Equivalence Assessment takes the adult equivalent biomass of larval and juvenile fish, along with the biomass of adult fish predicted to be entrapped, to estimate number of predators or the proportion of the predator
Although there have been previous efforts to quantify the loss of food resource to predators, these are limited and have rarely been applied within environmental assessments. Consequently, the indirect effect of fish Figure 1. Relationship between Equivalent Adult Values and entrapment to higher trophic levels is often considered on a theoretical or qualitative basis, Dietary Equivalence Assessments relying on the premise that if the direct effect of entrapment Entrapment of adult fish Entrapment of larval and (biomass per annum) juvenile fish on fish populations (number per annum) is demonstrated to Species-specific life be negligible, then history data (e.g. reproductive and indirect effects must mortality parameters) also be negligible and Equivalent Adult Value (EAV) of larval and juvenile fish therefore not significant. (number per annum) However, whilst it may be reasonable Species-specific weight to conclude a non @ 50% maturity significant direct effect if the predicted level Weight of equivalent adults (biomass per annum) of fish mortality from entrapment was considered to fall within the carrying Dietary Equivalent Assesment capacity of wider (i.e number of predators or the proportion of the predator Bioenergetics, dietary populations, this may population that would be sustained annually by the and demographic data biomass of fish prey entrapped) not necessarily translate into a negligible
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