Evaluating energy flows through jellyfish and gulf menhaden (Brevoortia patronus) and the effects of fishing on the northern Gulf of Mexico ecosystem

Abstract

Fishery management production models tend to stress only the elements directly linked to fish (i.e. fish, fish food, and fish predators). Large coastal jellyfish are major consumers of plankton in heavily fished ecosystems; yet, they are frequently not included as model components. We explore the relationship between gulf menhaden (Brevoortia patronus) and the large scyphozoan jellyfish (Aurelia spp. and Chrysaora sp.), and provide an examination of trophic energy transfer pathways to higher trophic levels in the northern Gulf of Mexico. A trophic network model developed within the ECOPATH framework was transformed to an end-to-end model to map foodweb energy flows. Relative changes in functional group productivity to varying gulf menhaden consumption rates, jellyfish consumption rates, and forage fish (i.e. gulf menhaden, anchovies, and herrings) harvest rates were evaluated within a suite of static, alternative energy-demand scenarios using ECOTRAN techniques. Scenario analyses revealed forage fish harvest enhanced jellyfish productivity, which, in turn, depressed menhaden productivity. Modelled increases in forage fish harvest caused pronounced changes in ecosystem structure, affecting jellyfish, marine birds, piscivorous fish, and apex predators. Menhaden were found to be a more efficient and important energy transfer pathway to higher trophic levels compared with jellyfish. A simulated increase in jellyfish abundance caused the relative production of all model groups to decline. These outcomes suggest that jellyfish blooms and forage fish harvest have demonstrable effects on the structure of the northern Gulf of Mexico ecosystem.

Jellyfish and forage fish spatial overlap on the eastern Bering Sea shelf during periods of high and low jellyfish biomass

“…Areas of no, medium and high overlap between Chrysaora melanaster and capelin Mallotus villosus. Shown are sampling stations (crosses), occurrence of capelin (green area), overlap between capelin and jellyfish (yellow area) and region of maximum overlap (> 75th percentile of biomass of both capelin and jellyfish; red area) larvae, physically harm fish via stinging nematocysts and affect the food web as a whole by diverting plankton production away from upper trophic levels (Brodeur et al 2008b, Ruzicka et al 2012, Robinson et al 2015. In the North Sea, there are inverse correlations between jellyfish biomass and Atlantic herring Clupea harengus recruitment.…”

Dietary overlap between jellyfish and forage fish in the northern Gulf of Mexico

“…Food web models have identified scyphomedusae and forage fish as key pathways of energy transfer within coastal food webs. Forage fish transfer energy to higher trophic levels, while scyphomedusae appear to direct energy toward the microbial loop or the benthic food web (Condon et al 2011, Robinson et al 2015. Recent studies have highlighted that the contribution of scyphomedusae to the dietary composition of their predators may have been underestimated (Utne-Palm et al 2010, Cardona et al 2012, D'Ambra et al 2015.…”

Comparing the roles of Pacific halibut and arrowtooth flounder within the Gulf of Alaska ecosystem and fishing economy

“…Trophic matrix A cp is expanded to include nutrient and detritus pools and account for the distribution of all consumption by group p between its consumers, between nutrient and detritus pools via feces and ammonium excretion, or to detritus as senescence. A model expressed in this format can readily be used to quantify the consequences of changes to community composition (Robinson et al., ; Ruzicka et al., ), changes to external subsidies of nutrients and plankton (Treasure, Ruzicka, Moloney, Gurney, & Ansorge, ; Treasure, Ruzicka, Pakhomov, & Ansorge, ), changes in oceanographic regime through coupled physical models (Ruzicka et al., , ), changes in fishery management policy, or changes to the physiology or diet of any functional group.…”

“…ECOTRAN models are based on the transformation of the solution for a system of linear equations describing predation pressure upon all members of a food web, such as solved by ecopath, into a donor-driven trophic matrix A cp that maps the fate of all production by groups p through the food web to consumers c (Steele, 2009;Steele & Ruzicka, 2011): where matrix D pc is the fraction of each producer p within the diet of each consumer c, q c is the total consumption rate of consumer c, and term ∑ c D pc q c is the total grazing or predation rate upon each producer p by all consumers c. Trophic matrix A cp is expanded to include nutrient and detritus pools and account for the distribution of all consumption by group p between its consumers, between nutrient and detritus pools via feces and ammonium excretion, or to detritus as senescence. A model expressed in this format can readily be used to quantify the consequences of changes to community composition (Robinson et al, 2015;Ruzicka et al, 2012), changes to external subsidies of nutrients and plankton (Treasure, Ruzicka, Moloney, Gurney, & Ansorge, 2015;Treasure, Ruzicka, Pakhomov, & Ansorge, 2018), changes in oceanographic regime through coupled physical models (Ruzicka et al, 2016, changes in fishery management policy, or changes to the physiology or diet of any functional group.…”