when concentrations of copepod nauplii were only 7.5 L−1, although first‐feeding was delayed up to four days compared to larvae provided prey at 120 L−1 (Kiørboe et al. 1985). Arula et al. (2015) analysed feeding conditions in Baltic C. harengus and found that they were substantially poorer during the first 10 days after larvae initiated feeding relative to conditions encountered by older larvae.
In the clupeid S. sprattus, nutritional condition and survival of larvae were correlated with the abundance of copepod prey (copepodid stages) in the Baltic Bornholm Basin (Voss et al. 2006) where growth rates were low when prey concentrations were <5 L−1 but were near maximum values at 20 L−1 (Hinrichsen et al. 2010). Physiological modelling of first feeding S. sprattus larvae suggested that prey concentrations of 30–40 copepod nauplii L−1 may be required for survival, but these threshold values were strongly dependent upon temperature (Peck & Daewel 2007). In the North American engraulid Anchoa mitchilli, substantial survival of larvae of this estuarine fish was reported when copepod nauplii concentrations exceeded 25 L−1 (Houde 1978). Substantial survival of the moronid Morone saxatilis and the alosine Alosa sapidissima larvae in Chesapeake Bay (USA) may occur if prey levels equal or exceed 50 L−1 (Houde et al. 1996, Leach & Houde 1999).
Prey patchiness at various spatial scales, or processes that elevate encounter rates between larval fishes and prey (e.g. microturbulence), has been proposed to explain larval feeding success at low prey concentrations (Lasker et al. 1970, Lasker 1975, Hunter 1981, MacKenzie et al. 1994). Mechanisms that increase availability of prey generally depend on a combination of physical processes, e.g. aggregating, concentrating, retaining (Bakun 1996) and larval behaviours that elevate the probability of encounter with prey. In estuaries, aggregating mechanisms and larval behaviour clearly can raise consumption levels and support higher growth rates. It also is now apparent that the high prey concentrations once thought necessary to ensure feeding success by fish larvae in marine systems were in part artefacts resulting from inadequate sampling or unreliable experimental conditions (Peck et al. 2013, Houde 2016). Furthermore, as knowledge of diets and feeding has increased, it is apparent, as noted above, that a wider variety of prey types is available to fish larvae than previously known.
Larval fishes must consume a ration that constitutes a large fraction of their body weight to account for their reported mean growth rates (Houde & Zastrow 1993). Llopiz (2013) estimated that median feeding incidence (percent of larvae with food in gut) of estuarine fish larvae was ~43%, a value much lower than the >90% median value for coastal and oceanic taxa. However, only 12 studies of feeding incidence by estuarine fish larvae were included in Llopiz's (2013) synthesis and he cautioned that the result might be misleading. In a synthesis of larval feeding data, Houde & Zastrow (1993) reported that the mean, temperature‐adjusted ingestion (i.e. daily ration, percentage of body weight) required for estuarine fish larvae to support mean reported growth rates was 68%, a value higher than the 57% estimated for larvae of all marine fishes. Estuarine fish larvae (and all fish larvae) have a high food demand, typically requiring a daily ration exceeding 50% of body weight, lending support to hypotheses invoking food as an important limiting factor for larval survival.
Based on Houde & Zastrow's (1993) synthesised data, the across‐taxa, weight‐specific, ingestion (I) versus temperature (T) relationships for marine and estuarine fish larvae are:
Accordingly, required weight‐specific ingestion for marine and estuarine larvae increases about 3–4% per degree increase in temperature, indicating that a substantially larger investment in feeding is required by larvae from low latitudes. Predicted required ingestions, expressed as percent of larval body weight, by estuarine fish larvae at 8, 18 and 28 °C are: 23, 63 and 104%, respectively. For some taxa, observed consumption is higher than predicted by these general relationships. For example, in laboratory experiments, daily consumption by larvae of the engraulid Anchoa mitchilli was >200% of their body weight at temperatures of 26–28 °C (Houde & Schekter 1981) at prey concentrations that occur in estuaries.
Ontogenetic shifts and feeding success
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
