The impacts of fish body size changes on stock recovery: a case study using an Australian marine ecosystem model

ICES Journal of Marine Science ICES Journal of Marine Science (2015), 72(3), 782– 792. doi:10.1093/icesjms/fsu185 Original Article The impacts of fish body size changes on stock recovery: a case study using an Australian marine ecosystem model Asta Audzijonyte 1,2*, Elizabeth A. Fulton 2, and Anna Kuparinen 1 1 Department of Environmental Sciences, University of Helsinki, Viikinkaari 2, PO Box 65, Helsinki FIN-00014, Finland CSIRO Oceans and Atmosphere Flagship, GPO Box 1538, Hobart Tasmania 7001, Australia 2 *Corresponding author: tel: +358 9 191 58443; fax: +358 9 191 58754; e-mail: asta.audzijonyte@helsinki.fi Audzijonyte, A., Fulton, E. A., and Kuparinen, A. The impacts of fish body size changes on stock recovery: a case study using an Australian marine ecosystem model. – ICES Journal of Marine Science, 72: 782 – 792. Received 17 June 2014; revised 25 September 2014; accepted 2 October 2014; advance access publication 3 November 2014. Many fished stocks show long-term reductions in adult body size. Such changes could lead to new feeding interactions and alter stock productivity, introducing new levels of uncertainty in fisheries management. We use a marine ecosystem model parameterized for Southeast Australia to explore how reductions (up to 6% in 50 years) in size-at-age of fished species affect stock recovery after an implementation of a fishing moratorium. We show that reduction in body size can greatly elevate predation mortality and lower the post-fishing biomass of affected species. In our simulations, the recovery period after the fishing moratorium was characterized by two phases. In the initial readjustment phase, the ecosystem dynamics was largely determined by the rapid changes in the biomasses of recovering species and changes in body size had negligible effects. In contrast, fish body sizes had the major impact on the biomasses in the second, semi-equilibrium state and the final biomasses were generally not affected by the harvest rate during the fishing period. When reduced size-at-age elevated predation mortality in most age groups of a species (tiger flathead Platycephalus richardsoni or silver warehou Seriolella punctata in our simulations), the species’ equilibrium biomass was considerably lower compared with the scenarios of no change in body size. For other species (pink ling Genypterus blacodes and jackass morwong Nemadactylus macropterus), a predation increase in some age groups was balanced by the decrease in others. The latter reduction in predation mortality occurred when major predators of species with reducing size-at-age were decreasing in size themselves, or when cannibalism was an important source of juvenile mortality (in blue grenadier Macruronus novaezelandiae). We suggest that decreased size-at-age will be most detrimental to stock recovery when the main predators of the stock are not affected by the drivers causing changes in body size. Keywords: ecosystem modelling, fisheries management, life history change, natural mortality, stock recovery. Introduction Many commercially exploited fish species have experienced reductions in abundance over the past decades (Hutchings, 2000; Mullon et al., 2005; but see Carruthers et al., 2012). The general theory of fisheries and density-dependent population growth suggest that, after reducing fishing pressure, fish stocks will bounce back rapidly due to relaxed intraspecific competition (Hilborn and Walters, 1992). This traditional view has lately been challenged by the observation that many overfished stocks have shown little signs of recovery despite dramatic reductions in fishing pressure (Hutchings and Reynolds, 2004; Neubauer et al., 2013). Generally, poor recovery has often been seen in large predatory fish with long lifespans such as cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), [although some cod stocks have recently shown good recovery, see Eero et al. (2012) or Kjesbu et al. (2014)], whereas small early maturing species have higher recovery capacity (Hutchings and Rangeley, 2011; Hutchings et al., 2012). Uncertainty around the productivity and recovery ability of fisheries stocks, and the main drivers behind this uncertainty, are a focus of fisheries science (e.g. Neubauer et al., 2013; Kuparinen et al., 2014). Several mechanisms have been suggested, possibly operating simultaneously. First, fishing can alter foodweb structure and trophic dynamics, particularly if selectively removing predatory fish at high trophic levels (Pauly et al., 1998; Garcia et al., 2012). Such directional removal of certain species can alter and even reverse predator–prey dynamics, whereby a decline in predator species leads to increased abundance of its prey, and this can then # International Council for the Exploration of the Sea 2014. All rights reserved. For Permissions, please email: 783 Impacts of fish body size changes on stock recovery further suppress the predator’s abundance if the adult prey species feeds on the fry or juveniles of the predator (Swain and Sinclair, 2000). Second, harvesting can also lead to life history changes of fished species. Many harvested stocks now mature at younger ages and smaller sizes, invest more energy in reproduction, and attain a smaller adult size-at-age (Sharpe and Hendry, 2009; Audzijonyte et al., 2013a). Mechanisms underlying such reductions are still being debated (e.g. Jørgensen et al., 2007; Kuparinen and Merilä, 2007; Swain et al., 2007; Heino et al., 2008; Dunlop et al., 2009), but an increasing amount of evidence suggests that these trends reflect, at least partially, fisheries-induced evolution [see Jakobsdóttir et al. (2011) and Therkildsen et al. (2013) for evidence on how fishing correlates to genetic changes in wild stocks]. Third, a similar trend of decreasing adult size can also be caused by temperatures warming above the species optimum (Cheung et al., 2012), as in, for example, eight North Sea commercial fish species (Baudron et al., 2014). Regardless of whether the life history changes are caused by climate change or fisheries-induced evolution, their ecological effects are likely to be similar with respect to the fact that they cause differential effects across species. This will lead to changes in the size spectra of fish communities. Decreasing size of reproducing individuals might reduce recruitment success (Birkeland and Dayton, 2005), increase natural mortality via survival costs of reproduction (Jørgensen and Fiksen, 2010), and, thereby, increase recruitment variability through reductions in longevity (Longhurst, 2002). At the ecosystem level, reductions in body size can also alter predator–prey dynamics through changes in relative body sizes of predators and preys and by increasing the time that prey spend at body sizes most vulnerable to predation (Audzijonyte et al., 2013b; Jørgensen and Holt, 2013). Because life history changes in exploited fish stocks are commonplace (Sharpe and Hendry, 2009), their role in species and (...truncated)