Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 58

Considered in the context of Hjort's (1914) critical period hypothesis, the period between yolk nutrition and transition to exogenous feeding may indeed be a critical time for larvae of estuarine fishes. As noted above, estuarine fish larvae must consume substantial quantities of plankton and require increasing amounts of prey during ontogeny and growth. Both numbers and sizes of prey generally increase as larvae grow, although larvae may continue to include small prey in the diet (Houde 2016). Feeding incidence initially is low but increases as larvae grow and develop, potentially reducing the threat of starvation while supporting fast growth. With increased mouth size, the spectrum of prey types and sizes available for consumption expands. There typically is a direct relationship between size of fish larvae and size of ingested prey (Houde 1997a, Houde 2016).

Sizes of prey that are eaten by marine and estuary‐associated fish larvae may shift during ontogeny because larvae are gape‐limited predators. Preferred prey of fish larvae usually is in the range ~2–10% of larval body lengths (Llopiz 2013). For example, in Chesapeake Bay, prey size (mostly copepod stages) in diets of the engraulid Anchoa mitchilli increases rapidly as larvae grow (Figure 3.12). First‐feeding larvae (~3 mm TL) consume prey of ~0.05 mm length (range 0.03–0.13 mm), while advanced, flexion‐stage larvae (~15 mm TL) consume prey of ~0.75 mm length, on average (range 0.08–1.30 mm). In this case, relative prey size increases from 1.7 to 4.8% of larval length as larvae grow from 3.5 to 15.0 mm TL (Figure 3.12).

In many cases, larvae consume and select bigger prey, on average, as they grow but continue to eat small prey, a feeding strategy that ensures sufficient calorie intake to support fast growth. For example, size of prey eaten by the clupeid Clupea harengus in the Baltic Sea increases as larvae grow (Hudd 1982), but larger larvae also eat smaller organisms (Arula et al. 2012a). Although large C. harengus larvae consumed small prey in the Blackwater Estuary (UK), these small prey, for example copepod nauplii, were not preferred (Fox et al. 1999). The size of C. harengus larvae at which preference shifts to larger prey may vary. Past research indicated that a shift to large prey occurred at 12–17 mm length (Bainbridge & Forsyth 1971, Checkley 1982). Recently, it was demonstrated that even small, first‐feeding C. harengus larvae in the Baltic's Gulf of Riga consumed some larger prey, e.g. copepodids and adult copepods (Arula et al. 2012a).

Figure 3.12 Relative lengths of prey (%) and actual lengths of prey (y‐axis) consumed by larvae of Anchoa mitchilli in five length bins

(modified from Auth 2003 and Houde 2016, his figure 3.26).

Figure 3.13 Prey size and niche breadth (defined here with respect to variability in sizes of prey) eaten by larvae of the moronid Morone americana (a, b) and the gobiid Gobiosoma bosc (c, d). Prey size PL (=prey length, μm) increases significantly with larval length in both species (a, c) but niche breadth S, the relative variability in prey size (standard deviation of mean prey length) increases with larval length for G. bosc (d) but not M. americana (b)

(derived from Campfield (2004)).

During ontogeny, niche breadth (defined here as the relative variability of prey sizes in the diet) may increase or remain constant, or even decline in some cases (Pepin & Penney 1997, Llopiz 2013). If niche breadth expands, in theory this signals a wider range and greater availability of suitable prey to benefit survival and growth of fish larvae in a prey‐limited environment. In one example, larvae of the moronid Morone americana in the Patuxent tidal sub‐estuary of Chesapeake Bay consumed larger prey as length increased but niche breadth did not change significantly during the larval stage (Figure 3.13). In contrast, size of prey and variability in prey sizes (i.e. niche breadth) did increase in a gobiid Gobiosoma bosc (Campfield 2004, Campfield & Houde 2011). In the estuary‐associated pleuronectid Pseudopleuronectes americanus, niche breadth did not increase as larvae grew (Pepin & Penney 1997), but it did increase for 6 of 10 other species (all continental shelf species) that were examined.

An increase in niche breadth was clearly observed in the gut analysis of larvae of the clupeid Sprattus sprattus in the Baltic Sea (Peck et al. 2012a and references therein). As larval size increased, prey size also increased and, based on analysis of combined data from different studies (Voss et al. 2003, Dickmann et al. 2007), prey size in S. sprattus increased most rapidly between 10 and 15 mm SL. At lengths >15 mm SL, mean and maximum prey sizes eaten by S. sprattus larvae changed relatively little (Last 1987, Bernreuther 2007), but the high variance in prey sizes indicated continued inclusion of small prey in the diet, evidence of an increase in niche breadth that is potentially important to insure fast growth. Similarly, Costa & Elliott (1991) demonstrated that with growth inside the Forth Estuary (Scotland) there was an increase in size of prey and the change from small‐to‐medium crustaceans and then to small fishes in diets of the juvenile gadoids Gadus morhua and Merlangius merlangus.

During critical transitions in early development, for example metamorphosis in some fishes, feeding ability and success also may be diminished because of ontogenetic changes, often accompanied by shifts in habitat and settlement (see Section 3.3.2). These probable stresses may be particularly important in estuary‐associated pleuronectiforms in which dramatic changes in morphology occur (Able & Fahay 2010). In the lateolabracid Lateolabrax japonicus, there is evidence of reduced feeding success during metamorphosis that may be a factor affecting recruitment (Islam & Tanaka 2005). In sciaenid fishes, shifts in diet are associated with development of the jaw during metamorphosis (Figure 3.6) and are accompanied by shifts in habitat in pelagic and benthic sciaenid species (Deary et al. 2017). Although diets of three sciaenids in Chesapeake Bay were similar during pelagic, early‐larval stages, the diets diverged during metamorphosis (at 17–20 mm) (Deary et al. 2017).