Effects of Hypoxia on Consumption, Growth, and RNA:DNA Ratios of Young Yellow Perch

As in various freshwater and coastal marine ecosystems worldwide, seasonal bottom water hypoxia is a recurring phenomenon in Lake Erie’s central basin. While bottom hypoxia can strongly affect sessile benthic animals, its effects on mobile organisms such as fish are less understood. We evaluated the potential for bottom hypoxia to affect the growth rates of yellow perch Perca flavescens, a species of ecological and economic importance in the lake. To this end, we (1) conducted laboratory experiments to quantify the effects of reduced dissolved oxygen on consumption, somatic growth, and RNA:DNA ratios (an index of short-term growth) of young yellow perch and (2) explored the effects of bottom hypoxia on young yellow perch growth in Lake Erie’s central basin by collecting individuals in hypoxic *Corresponding author: [email protected] 1Present address: Department of Fish, Wildlife and Conservation Biology, Colorado State University, 1484 Campus Delivery, Fort Collins, Colorado 80523, USA. Received August 20, 2010; accepted May 19, 2011 1574 D ow nl oa de d by [ C ol or ad o St at e U ni ve rs ity ] at 0 8: 36 0 7 D ec em be r 20 11 HYPOXIA ON CONSUMPTION, GROWTH, AND RNA:DNA OF YELLOW PERCH 1575 and normoxic regions of the lake and quantifying their RNA:DNA ratios. Yellow perch consumption and growth in our experiments declined under hypoxic conditions (≤2 mg O2/L). While yellow perch RNA:DNA ratios responded strongly to experimental temperature, nucleic acid ratios were not significantly affected by dissolved oxygen or feeding ration. We did, however, observe a positive correlation between yellow perch growth and RNA:DNA ratios at low temperatures (11◦C). The nucleic acid ratios of yellow perch collected in Lake Erie varied spatiotemporally, but their patterns were not consistent with hypoxia. In short, while yellow perch consumption and growth rates respond directly and negatively to low oxygen conditions, these responses are not necessarily reflected in RNA:DNA ratios. Moreover, in central Lake Erie, where yellow perch can behaviorally avoid hypoxic areas, the RNA:DNA ratios of yellow perch do not respond strongly to bottom hypoxia. Thus, this study suggests that there is no strong negative effect of bottom hypoxia on the growth of young yellow perch in Lake Erie. Hypoxia (≤2 mg O2/L) is a recurring phenomena in both marine and freshwater ecosystems worldwide (Diaz 2001; Diaz and Rosenberg 2008). Previous studies have demonstrated that hypoxia can influence fish distributions, foraging behavior, and food web interactions both directly and indirectly (e.g., Pihl 1994; Eby and Crowder 2002; Craig and Crowder 2005; Roberts et al. 2009). However, the direct and indirect effects of hypoxia on fish growth are less clear (but see Stewart et al. 1967; Brandt et al. 2009). While laboratory experiments have demonstrated the direct effects of low oxygen concentrations on fish physiology (Kramer 1987; Thomas et al. 2005; Stierhoff et al. 2006), the effects of hypoxia are less evident in natural ecosystems because many fishes can detect and avoid hypoxic areas. For example, in the laboratory low dissolved oxygen concentrations may lead to responses such as increased ventilation (Petrosky and Magnuson 1973; Suthers and Gee 1986; Ludsin et al. 2009) and can negatively influence food consumption (hereafter referred to as consumption) and growth of fishes (e.g., Stewart et al. 1967; Brandt et al. 2009). In natural systems, avoidance of hypoxic conditions can influence distributions of fishes (e.g., Suthers and Gee 1986; Ludsin et al. 2009) and indirectly affect diet patterns (Pihl 1994; Aku et al. 1997; Taylor et al. 2007). While such effects on diet and movement of fish into novel habitats may be expected to influence growth, few field studies have demonstrated hypoxia-induced growth consequences for fish in natural systems (e.g., Eby et al. 2005; Stierhoff et al. 2009). Traditional methods of examining fish growth (e.g., chronometric structures) integrate responses to environmental conditions over long periods (e.g., annually, lifetime). Such methods are, however, not ideal for examining growth responses to hypoxia, given the relatively short persistence (hours to months) of hypoxia in most systems. Nucleic acid ratios (RNA:DNA ratios) are useful indices of short-term growth and have been used to examine short-term growth of various aquatic organisms such as zooplankton (Gorokhova and Kyle 2002; Gorokhova 2003), larval fish (Clemmesen 1994; Pepin et al. 1999; Höök et al. 2008), and juvenile and adult fish (Gwak et al. 2003; Smith and Buckley 2003; Stierhoff et al. 2009) including yellow perch Perca flavescens (Audet and Couture 2003; Tardif et al. 2005; Glemet and Rodriguez 2007), our study species. Whereas DNA concentrations in cells are relatively static, RNA concentrations are more dynamic with greater amounts of RNA indicating increased protein synthesis and therefore increased growth (Bulow 1987). Ratios of RNA to DNA are sensitive to temperature because for most fish species, more RNA is required at lower temperatures than at higher temperatures to induce similar metabolic and growth rates (Buckley 1982; Goolish et al. 1984). Several laboratory studies have developed speciesand temperature-specific positive relationships between growth and RNA:DNA ratios (Clemmesen 1994; Grant 1996; Ali and Wootton 2003; MacLean and Caldarone 2008; Stierhoff et al. 2009), thereby facilitating a positive linkage between RNA: DNA ratios and somatic growth. We focused our study on the growth consequences of hypoxia for yellow perch in Lake Erie’s central basin (LECB). Yellow perch are an ecologically and economically important fish species in LECB (Ryan et al. 2003), a system that frequently experiences hypolimnetic hypoxia during late summer (Edwards et al. 2005; Hawley et al. 2006; Rao et al. 2008). Carlson et al. (1980) demonstrated through laboratory experiments that yellow perch growth declines when the dissolved oxygen concentration approaches 2 mg/L at about 20◦C. Thus, while prolonged, direct exposure of yellow perch to low oxygen should negatively affect growth at 20◦C, the direct effects of low dissolved oxygen on yellow perch consumption, growth, and RNA:DNA ratios at diverse temperatures remain understudied. Elucidating the ecological impacts of hypolimnetic hypoxia in LECB is particularly important because hypoxia recurs annually in Lake Erie and can cover a very large portion of the basin (in 2005 the hypoxic zone was estimated to cover 10,000 km2, Hawley et al. 2006). A recent field study demonstrated that bottom hypoxia could alter patterns of habitat use (i.e., reduced use of bottom waters) and foraging (i.e., reduced consumption of benthic macroinvertebrates and increased consumption of zooplankton) of yellow perch in LECB (Roberts et al. 2009). Increased occupation of relatively warm, epilimnetic waters should increase metabolic demands, and increased consumption of small, energetically inferior zooplankton prey may result in reduced growth rates. Thus, we hypothesized that bottom hypoxia should also indirectly negatively affect growth of yellow perch in LECB. D ow nl oa de d by [ C ol or ad o St at e U ni ve rs ity ] at 0 8: 36 0 7 D ec em be r 20 11