ICES CM 2013/E:32 (POSTER)

Ocean acidification and the possible loss of benthic : would commercial fin- be affected? John K. Pinnegar, Bryony L. Townhill, Silvana N.R Birchenough, Steven Mackinson, William J.F. Le Quesne Centre for Environment, Fisheries & Aquaculture Science, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK. Contact details: e-mail [email protected], phone +44 1502 524229.

Summary Ocean acidification (OA) has been suggested as posing a major threat to commercial fisheries worldwide, both directly through physiological impacts (e.g. on commercial shellfish) but also indirectly through ‘bottom-up’ influences on higher trophic-level organisms including important fin- fish . Echinoderms and bivalve molluscs are thought to be particularly vulnerable to low pH as a result of their high-magnesium skeletons. In the present analysis we have demonstrated that these benthic organisms are important prey resources for dab, haddock, and plaice in the North Sea and that predation on these benthic invertebrates occurs throughout the whole region and in every season of the year.

A hypothetical analysis using the food-web model Ecopath with Ecosim (EwE) has revealed that it is often be very difficult to deduce from first-principles, the likely consequences of removing echinoderms or bivalve molluscs, in terms of ‘bottom up’ impacts on fin-fish biomass and hence on fishery yields. Most fin-fish species are able adapt their diet to target other prey types (e.g. ) when echinoderms or bivalve molluscs disappear. Consequently although the model suggests that certain fin-fish may be negatively impacted by OA (most notably wolfish, rays, small sharks etc), other species are predicted to persist or even proliferate (e.g. plaice, lemon sole, sole), and this could result in unforeseen positive consequences for fisheries.

Introduction So far, few studies have attempted to investigate the potential “bottom up” impacts of ocean acidification on marine food-webs, and hence on fisheries (see Le Quesne and Pinnegar, 2012). Several authors have hypothesised that such effects could be substantial (Cooley and Doney 2009), and at least as important as ‘direct’ physiological impacts in some instances. Laboratory experiments have tended to suggest that echinoderms and certain molluscs are among the most directly vulnerable of organisms to ocean acidification, largely because their skeletons are composed of high magnesium calcite that is 30 times more soluble in comparison with low magnesium calcite or aragonite and thus is particularly susceptible to dissolution as ocean pH decreases (Dupont et al 2010).

Methods In order to quantify the absolute importance of echinoderms and bivalves as a prey resource for fin- fish in the North Sea we examined fish stomach content data from the newly-compiled Cefas ‘DAPSTOM’ database that includes data from 286 distinct research cruises spanning the period 1893- 2010. Diet composition data were available for 71 predator species (72,169 stomachs), with haddock, plaice, dab, dragonet, and wolfish shown as being particularly reliant on echinoderms and bivalve molluscs. In order to ‘scale up’ to absolute estimates of ‘importance’ we made use of biomass estimates (g/m2) as well as consumption rates (g/m2/yr) for each fish species from Mackinson and Daskalov (2007).

Mackinson and Daskalov (2007) constructed a complex food-web model for the North Sea using Ecopath with Ecosim (EwE) software. This model incorporates 68 functional groups and 12 fishing

1 | P a g e fleet categories defined by EU Data Collection Regulations. Nearly all echinoderm and bivalve species are currently included in the model as ‘Epifaunal macrobenthos’, although the most abundant species Amphiura filiformis (42% of the total biomass) is contained within the “Infaunal macrobenthos” group. The impact of OA was simulated by driving the biomass of each benthic group downward over time using the in-built ’fitting’ routine. A number of ’forcing’ scenarios were tested, firstly a 1% year-on- year decrease in the biomass of each group but also a 0.5% decrease for the full 100 years. A 1% decline year on year would equate to an effect size over the 100 year duration of 0.63 (expressed relative to the ‘control’ year). Similarly a 0.5% year on year decline would equate to an effect size of 0.39, broadly comparable in magnitude to those described by Dupont et al (2010).

Results Echinoderms do not represent a major component of the diet for fish species, achieving 35% in dragonet and 16% in dab but less than 10% in all other species. The vast majority of the food consumed by benthic-feeding fish predators in the North Sea seems to comprise bivalve molluscs, crustaceans and worms – although this varies from region to region, year to year and season to season. Dab are by far the most abundant fish in the North Sea region, and hence – even though echinoderms represent a very small part of their diet, this scales to a very large number of individual organisms consumed each year (around 8,172,488 million). More echinoderms are consumed by dab than by all other predators combined, even though the numbers consumed by haddock, dragonet and long-rough dab are also considerable. When decreases in both benthic groups were implemented simultaneously, the impacted groups declined by 49.1 and 85.7 t/km 2 under the 1% scenario. This resulted in very large increases in the biomass of some commercial fin-fish (table 1), in particular lemon sole, plaice, dover sole and dragonet, but also modest declines in small sharks, thornback/spotted ray, adult saithe, Norway pout and large crabs. These changes had dramatic consequences for predicted fishery yields and revenues. The fishing fleet that appeared to benefit the most was the beam trawl fleet – largely because of Table 1. Relative biomass of each functional group (year 100/year 0) in a predicted increase in flatfish, response to a 1% year on year decline in epifaunal and infaunal their main target species. macrobenthos. Red –losers, Blue –winners, Black no significant change.

References Cooley, S.R., and Doney, S.C. 2009. Anticipating ocean acidification’s economic consequences for commercial fisheries. Environmental Research Letters, 4: 024007 8pp. Dupont, S., Ortega-Marinez, O. and Thorndyke, M. 2010. Impact of near-future ocean acidification on echinoderms. Ecotoxicology, 19: 449-462. Le Quesne, W.J.F. and Pinnegar, J.K. 2012. The potential impacts of ocean acidification: Scaling from physiology to fisheries. Fish and Fisheries, 13: 333-344. Mackinson, S. and Daskalov, G., 2007. An ecosystem model of the North Sea to support an ecosystem approach to fisheries management. Science Series Technical Report, Cefas Lowestoft, 142: 196pp.

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