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).
Nutritional considerations
Both the amounts and nutritional quality of food are important to ensure growth of estuary‐dependent fish larvae. While amounts may be most critical, quality potentially can significantly affect growth and survival although it is difficult to evaluate. Approaches to evaluate nutritional status of estuarine fish larvae under differing environmental conditions, and when prey levels vary, are useful to investigate causes of variable growth and mortality. Biochemical methods, based on quantification of nucleic acids, have frequently been applied in recent decades to judge if larvae were feeding sufficiently to ensure survival. In a comprehensive analysis, Rooker et al. (1997) compared RNA:DNA ratios in postflexion larvae of a sciaenid Sciaenops ocellatus from different seagrass habitats in two Texas estuaries, finding that >95% of newly settled larvae were in good nutritional condition, indicating they were unlikely to starve. In two South African estuaries, RNA:DNA ratios, lipid levels and protein content were compared (Costalago et al. 2014) for postflexion larvae of the clupeid Gilchristella aestuaria, showing that RNA:DNA was an effective metric to detect differences in nutritional condition in the two estuaries. A similar approach applied to several taxa of estuarine fish larvae in Portuguese estuaries found that a nutritional condition index was positively correlated with abundance of plankton prey (Esteves et al. 2000), a finding supported in multiple studies in South African estuaries by Costalago et al. (2015) and Bornman et al. (2018) for larvae of Gilchristella aestuaria.
Analysing larvae of the pleuronectid Pseudopleuronectes americanus in laboratory experiments, Buckley (1980, 1984) reported that growth rates were faster at higher temperatures, but the RNA:DNA ratio was not temperature dependent. In larval Pleuronectes platessa, both growth rate and RNA:DNA ratio were positively correlated with temperature in the North Sea (Hovencamp & Witte 1991) before larvae ingressed to coastal and estuarine nurseries. Laboratory experiments on the larval moronid Morone saxatilis demonstrated that both growth and RNA:DNA ratio were highly correlated with food levels and feeding protocols (Wright & Martin 1985). Identifying causative factors that explain outcomes of RNA:DNA analyses on estuarine fish larvae can be difficult and subject to variability induced by many factors. However, in several field studies on coastal and estuarine fish larvae, the reported RNA:DNA ratios were high and indicative of good growth, suggesting that prey resources typically were adequate to fuel growth, or that selective predation might have eliminated larvae in poor nutritional condition.
Stable isotopes (SI) of carbon (δ13C), nitrogen (δ15N) and sulphur (δ32S) can be good indicators of diet sources, ontogenetic shifts in diet and potentially of nutritional condition in estuarine fish larvae (Hoffman et al. 2007). An analysis on recently transformed juveniles of the alosine Alosa sapidissima in the York River sub‐estuary of Chesapeake Bay (Virginia, USA) showed a gradual shift in δ13C and δ32S during ontogeny towards SI signatures of more marine sources as the juveniles migrated down‐estuary (Hoffman et al. 2007). In this example, the juveniles had a higher potential for recruitment in years of high freshwater discharge when the SI signature in δ13C indicated relatively high contributions of freshwater‐derived food. In the sparid Sparus aurata, carbon and nitrogen stable isotopes showed evidence of a clear shift in trophic position and trophic pathways during growth from the postlarval to juvenile stage upon ingress to the Lagoon of Venice, Italy, from offshore waters of the Adriatic Sea (Andolina et al. 2020). The shift was the result of a change in diet from zooplanktivory to zoobenthivory and a broader trophic niche in juvenile S. aurata feeding within the lagoon. In estuaries, unlike the sea, allochthonous carbon sources can vary on short timescales or inter‐annually, depending on environmental conditions, including variable seasonal precipitation and freshwater inflows that affect plankton productivity and community structure, as well as availability of prey types to fish larvae.
In an analysis of larvae of the moronid Morone saxatilis in Chesapeake Bay, inter‐annual differences in abundance and availability of two major prey organisms, a copepod Eurytemora carrolleeae (=affinis) and a cladoceran Bosmina sp., were demonstrated to differ spatially and effects on feeding by M. saxatilis larvae on these two prey types were apparent in their SI signatures (Shideler & Houde 2014). Additionally, the inter‐annual variability in SI signatures (levels of δ13C and δ15N) of adult spawner M. saxatilis, which can be a potential marker for nutritional well‐being, was registered in the SI values of newly hatched, yolk‐sac larvae. Stable isotope analyses, in addition to diet and nutritional evaluation of estuary‐dependent or ‐associated fish larvae, have value for tracking sources, dispersal, immigrations and connectivity of recruiting fish larvae and juveniles (Herzka et al. 2002, Herzka 2005).
3.3.4 Larval and juvenile production: growth and mortality
Success
