A MODEL OF THE EFFECTS OF LIMITED PREY AVAILABILITY ON THE SURVIVAL OF WEAKFISH (CYNOCION REGALIS) LARVAE.
KEY WORDS: weakfish, larvae, prey availability, survival
Recent work has demonstrated that mortality of fish larvae can be attributed to a combination of density-independent processes such as advective transport (Rowe and Epifanio, 1994), pollution (Tsai et al., 1991), and habitat destruction (Price et al., 1985), as well as density-dependent mechanisms such as competition (Duffy and Epifanio, 1994), predation (Jenkins et al., 1991; Dufly et al., 1997), and prey limitation (Cowan and Houde, 1990). Of all these factors, the contribution of prey limitation has been the most thoroughly studied, and yet remains the most controversial. While there has been a large body of laboratory evidence to suggest that prey availability and mortality are directly coupled (Miller et al., 1988; Gadomski and Petersen, 1988; Pederson et al., 1990; Goshorn and Epifanio 1991a). field evidence has been equivocal (Cowan and Houde, 1990; Duffy and Epifanio 1994). As such, the long-held theory that recruitment is a function of starvation-induced mortality among early life-history stages (Hjo rt, 1914; Cushing, 1972; Lasker, 1975) has come under close scrutiny, and the contribution of starvation mortality to total recruitment has been questioned (Leggett and DeBlois, 1994). Although there is little field evidence to link prey limitation with actual starvation of larvae, the indirect effects of prey limitation may still be important, and prey-mediated changes in growth rate, competitive ability, foraging success, and predator evasion may have effects on overall survival.
We developed a simple, deterministic model to examine the effects of prey availability, temperature, and mortality rate on the survival of a hypothetical cohort of weakfish larvae. While more complicated stochastic models are designed to closely simulate the overall natural environment, our model provides a detailed analysis of the sensitivity of a single process (growth rate) to realistic variation in a few key parameters. This approach offers an unequivocal set of results that allow a rigorous test of a defined set of otherwise qualitative ideas. Both stochastic and deterministic models have been developed to quantify some of the factors influencing larval mortality and thus predict changes in larval supply (Pepin, 1989; Rice et al., 1993; Chesney, 1993). In our model we utilized data from a series of field experiments with weakfish larvae (Duffy et al., 1996) to calculate critical rates of instantaneous mortality, i.e., rates of mortality at which cohort failure might be expected to occur.
Weakfish (Cynoscion regalis Bloch and Schneider: Sciaenidae) occur along the U.S. coastline from Florida to Massachusetts and spawn in shallow bays and estuaries in the Mid-Atlantic Bight from May through August (Taylor and Villoso, 1994). Spawning activity occurs in two pulses during the season (Lowerre-Barbieri et al., 1996). Larvae can begin feeding at three days post hatching (3 DPH) and continue to feed on zooplankton until they reach metamorphosis and become demersal (Goshorn and Epifanio, 1991a). Weakfish have historically supported large commercial and sport fisheries, and available catch records suggest that weakfish stocks vary at low frequency, i.e., on a scale of decades. The absence of high frequency variation in stock size may be related to the longevity of the species ([greater than] 10 y), but also suggests that density-dependent factors may be important regulators of recruitment in weakfish.
MATERIALS AND METHODS
Data for the model were obtained from a series of previously published field experiments that examined effects of prey limitation on growth and survival of weakfish larvae (Cope, 1991; Duffy et al., 1996). The early stage encompassed the period between first feeding (X dry weight = 10.8 [micro]g) and the development of partially differentiated fins rays and minute teeth (X dry weight = 36.9 [micro]g). Late stage was defined as the period between initial appearance of fin rays and the onset of metamorphosis (X dry weight = 179.3 [micro]g). Morphological characters associated with metamorphosis included a flexed notochord, fully developed fin rays, a pronounced chromatophore at the base of the caudal fin, and well developed teeth. Data for dry weights and morphological characteristics are derived from Duffy and Epifanio (1994), Duffy et al. (1996), and unpublished laboratory data collected over five consecutive spawning seasons.
Experiments (Cope, 1991; Duffy et al., 1996) were performed in field enclosures (1.4 [m.sup.3]) that allowed the movement of water while retaining weakfish larvae and their prey. In the early-stage experiment, weakfish larvae (2 DPH) were stocked in duplicate enclosures at a density of one larva [l.sup.-1]. Wild zooplankton (53-253 [micro]m) were stocked in the enclosures at each of three prey densities (10, 100, and 1000 prey [l.sup.-1]). During the weakfish spawning season, typical zooplankton densities in a Mid-Atlantic estuary probably fall within 50 -500 prey [l.sup.-1], though the formation of small-scale patches can contribute to observed higher levels (Goshorn and Epifanio, 1991 b), warranting the inclusion of a high (1000 [l.sup.-1]) prey density treatment. Experimental duration was one week, after which the enclosures were retrieved and the contents preserved. In the late-stage experiment, enclosures were stocked with laboratory-reared 9 DPH larvae. Prey treatments were the same, and the experiment al duration was again one week. In all experiments, growth was assumed to be exponential and characterized by an instantaneous rate coefficient, [G.sub.w].Results of the combined experiments demonstrated a significant effect of prey density on [G.sub.w].Over the range of prey densities tested, mean values for [G.sub.w] ranged from 0.14-0.21 [d.sup.-1] for early-stage larvae and from 0.07-0.19 [d.sup.-1] for late-stage larvae (Table 1).
The model consisted of an initial regression analysis that determined the relationship between measured values for [G.sub.w] from the enclosure experiments and the respective prey densities. In an earlier experiment, Cope (1991) found that instantaneous growth of weakfish larvae varied directly with the logarithm of prey density, and we fit our data to a similar equation:
[G.sub.w] = a ln (P) + b (1)
where a = slope of the regression line, b = y-intercept, and P = prey density. Thus, we were able to estimate [G.sub.w] for any naturally occurring prey density.
Model-generated values for [G.sub.w] were then used to calculate the respective durations of early-stage and late-stage larval development for any prey concentration. This was done by rearrangement of the standard equation describing exponential growth of larvae. For early-stage larvae this was:
[T.sub.E] = (ln [W.sub.E] - ln [W.sub.1]) [divided by] [G.sub.w] (2)
where [T.sub.E] = the time spent in the early-stage (days), [W.sub.I] = mean weight at first-feeding (10.8 [micro]g), [W.sub.E] = mean weight at the end of the early-stage (36.9 [micro]g), and [G.sub.w] = instantaneous growth rate derived from the regression analyses above. Duration of development of late-stage weakfish larvae ([T.sub.L]) was calculated in a similar manner ([W.sub.L] = 179.3 [micro]g) and total duration of larval development was determined by the sum of [T.sub.E] and [T.sub.L].
Exponential rates of mortality (Z) measured in the enclosure experiments (0.15 -0.36 [d.sup.-1]) were not significantly affected by variations in prey density and were generally lower than previously reported field values. This was probably due to the intentional exclusion of predators from the enclosures during the experiments. Therefore, we used simulated values for Z ranging from 0.1 to 1.0 [d.sup.-1], which is approximately equivalent to 10% to 60% mortality per day. This range of values for Z was based on previously reported field values for larvae of several species in the family Sciaenidae (Peebles and Tolley, 1988; Cowan et al., 1992). For any value of Z, the model calculated the number of larvae remaining at the conclusion of both the early and the late stage as:
[N.sub.T] = [N.sub.I] ([e.sup.-ZT] (3)
where [N.sub.I] = number of larvae at initiation of a given stage, [N.sub.T] = number of larvae at conclusion of the stage, Z = instantaneous mortality, and T = duration of a stage. Percent mortality (M) was expressed as:
% M = (1 - [e.sup.-ZT]) * 100 (4)
Therefore, the model gave not only the duration of larval development under conditions of varying prey density, but also the percent survival under varying conditions of simulated mortality.
We investigated the role of temperature on larval development and stage duration by estimating growth rates at a range of temperatures (17[degrees]C-25[degrees]C) that might occur in Delaware Bay during weakfish spawning season (Epifanio et al. Duffy and Epifanio, 1994; Rowe and Epifanio, 1994; Duffy et al. 1996). Our field prey density study was conducted at 19[degrees]C, so we used these growth rates and an estimate of [Q.sub.10] to approximate the growth rates of early-stage and late-stage larval weakfish at 17, 21, 23, and 25[degrees]C. We set [Q.sub.10] for larval weakfish at 2.4, an average value derived for juvenile weakfish at similar temperatures (Lankford and Targett, 1994; Hartman and Brandt, 1995). [Q.sub.10] values among larvae (X = 3.0) are often higher than those among juveniles and adults (X = 2.0) (Rombaugh 1988), so we examined the literature for values for larvae of other temperate estuarine species. We calculated the [Q.sub.10] for larval red drum, Sciaenops ocellatus, and determined it w as approximately 1.9 at temperatures 24-28[degrees]C (Lee et al., 1984). Similarly, we examined [Q.sub.10] values for larvae of Atlantic menhaden, Brevoortia tyrannus, (2.4 at temperatures 19-24[degrees]C), Atlantic herring, Clupea harengus, (3.0 at temperatures 13-18[degrees]C), and tautog, Tautoga onitis (1.9 at temperatures 16 - 22[degrees]C) (Rombough, 1988), and concluded that while our estimate for weakfish larvae was derived for studies conducted with juveniles, it fell within the expected range of [Q.sub.10] values for estuarine larvae and therefore provided a reasonable estimate of variations in growth rate with changes in temperature.
The model was used to calculate critical rates of instantaneous mortality ([Z.sub.CR]), i.e., rates of mortality at which cohort failure might be expected to occur. Literature data on cohort failure of weakfish larvae were unavailable. However, a theoretical exercise by Houde (1987) reported that for fecund species of fish, a two order-of-magnitude decrease in the number of viable yolk-sac larvae between the beginning and the end of the larval period resulted in successful recruitment. A three order-of-magnitude decrease in recruitment was considered poor. We chose to be conservative and deemed an unsuccessful recruitment, or "recruitment failure," as a four order-of-magnitude decrease between the initial and final numbers of larvae in the cohort. In our model simulations, we set [N.sub.1] at 1 x [10.sup.9] and defined [Z.sub.CR] as the rate at which [N.sub.T] [less than or equal to] 1 x [10.sup.5]. It should be noted however, that while a decrease of four orders of magnitude may be catastrophic for many fis h species, some species, such as the striped bass (Morone saxatilis), can sustain losses at this level and still produce a successful year class (Houde, 1997a).
Regression analysis showed a significant positive relationship between larval growth and prey density (Table 2). The duration of early-stage development varied from 6.0 days at 1000 prey [I.sup.-1] to 9.2 days at 10 prey [I.sup.-1], while late-stage development varied from 8.4 d to 16.5 d. Thus, overall time to metamorphosis varied from 14 to 26 days across the natural range of prey densities (Table 3).
The model predicted that small changes in Z resulted in large changes in percent survival to metamorphosis, a phenomenon that has been described elsewhere (Houde, 1987). For example, when Z varied from 0.35 to 0.40 [d.sup.-1] at 100 prey [I.sup.-1], percentage survival to metamorphosis declined from 0.16% to 0.06% respectively, a nearly 3-fold reduction (Table 4). Moreover, the model predicted [less than] 0.1% survival to metamorphosis at estimated rates of mortality for weakfish larvae in nature (see below). Prey density also had a strong effect on larval survival, particularly at high values of Z. When Z was held constant at 0.4 [d.sup.-1], the total number of survivors decreased nearly an order-of-magnitude among early-stage larvae and two orders-of-magnitude among late-stage larvae between the highest and lowest prey densities tested (Figure 1). Also, model simulations also showed that critical values of mortality ([Z.sub.CR]) varied with prey concentration and ranged from approximately 0.4 to 0.65 [d.sup.-1] at prey densities spanning three orders-of-magnitude (Figure 2).
Temperature also had a substantial effect on growth rates and therefore on levels of critical mortality ([Z.sub.CR]). At 17[degress]C, growth rates ([G.sub.w]) among weakfish larvae ranged from 0.11-0.19 [d.sub.-1], while at 25[degrees]C they ranged from 0.19-0.43 [d.sup.-1] across the range of prey densities used in the model. Critical mortality varied accordingly and rose from 0.17[degrees]C and 10 prey [l.sup.-1] to greater than 1.0 at 250[degrees]C and 1000 prey [l.sup.-1] (Figure 3).
Estimates of natural mortality for the larvae of estuarine and coastal fish generally range from 0.2-0.6 [d.sup.-1] (e.g., Peebles and Tolley, 1988; Houde, 1989; Rutherford and Houde, 1994). Our model predicts between 2.5 and 0.1% survival to metamorphosis for weakfish larvae at 7-values in this range. These results are similar to field estimates of survival of larval cohorts for five coastal species found over a wide latitudinal range in the western Atlantic (Houde, 1987).
The results of the model demonstrated that survival to metamorphosis was modulated by small changes in rates of mortality, and that slight increases in 7 had two- and three-fold impacts on the total number of survivors. Houde (1987) was among the first to consider such a possibility, and suggested that conditions which produce slight changes in rates of mortality could pose a more serious threat to the recruitment of marine fish larvae than catastrophic phenomena which destroy huge portions of the population at once (Houde, 1989). Other authors have supported this hypothesis (Chesney, 1989; Cowan and Houde, 1990; Jenkins et al., 1991; Duffy and Epifanio, 1994), and data from the present model provide further evidence.
In our analysis of critical mortality, the model predicted that [Z.sub.CR] could vary by as much as [0.4d.sup.-1] within the range of prey densities tested. Perhaps most interesting are model-derived effects at lower prey treatments, 50-500 zooplanktors [l.sup.-1], which reflect ambient densities for Mid-Atlantic estuaries. The most dramatic changes in the shape of the response surface occurred in these lower prey ranges, indicating that slight variations in prey density may have marked effects on overall cohort survival. Previous work has correlated growth rate with recruitment (Campana, 1996), however it has been argued that mortality may be the greatest contributor to recruitment variability (Hunter, 1981). Of course, it is generally believed that neither variations in [G.sub.w] or Z alone are responsible for fluctuations in year class strength, but that these two factors act jointly to affect recruitment (Cushing, 1975; Houde, 1987). As such, models that relate Z and G provide the most promising avenue f or pursuit. For example, the development of a stage-specific mortality index (mortality/growth ratio) (Houde, 1997 a, b) offers a practical method to predict changes in cohort biomass and its potential to affect recruitment.
The response surface derived from the model in the present study showed that critical rates of mortality for weakfish at 19[degrees]C could range from 0.4 to 0.5 [d.sup.-1]. Published estimates of field mortality for weakfish and other sciaenid larvae are in this range, which suggests that typical mortality rates for weakfish larvae in natural systems may be very near the critical value. The model showed that variations in temperature can exacerbate the effects of prey density on the magnitude of [Z.sup.CR]. For example, at a prey density of 10 [l.sup.-1], [Z.sup.CR] changes from 0.4 to 0.3 [d.sup.-1] with a drop in temperature from 19 to 17[degrees] C (Figure 3). This could have significant effects on cohorts of weakfish larvae at temperatures typical of Delaware Bay (17-21[degrees] C) during the early-season weakfish spawning peak. Similar effects have been observed in actual field studies with larvae of other species of fish. For example, Rutherford and Houde (1994) demonstrated that survival was much gre ater among striped bass, Morone saxatilis, cohorts hatched when temperatures were above 17[degrees]C, whereas cohorts subjected to temperatures below that demonstrated reduced survival and, in some cases, total cohort failure.
While the use of [Q.sup.10] relationships gives an adequate model response to low temperature, there may be additional deleterious effects of high temperature that are beyond the scope of this approach. For example, the model predicts that high temperatures will invariably increase growth rates and raise threshold levels of critical mortality. However, high temperatures raise general metabolic rates in fish larvae, and under conditions of low prey density and high temperature, larvae might not be able to consume enough food to compensate for the higher energy expenditure. Thus, energy available for growth would be reduced, resulting in protracted larval duration and reduced survival to metamorphosis for any given mortality rate.
Nevertheless, the overall results of the model indicate that weakfish larvae develop in an environment where the risk of failure of any particular cohort is high. If this model result is an accurate representation of the risk of cohort failure in nature, then we would expect that weakfish would have evolved appropriate life-history strategies in response. Ecological theory predicts that species reproducing in environments where the risk of reproductive failure is high should be bet-hedgers, i.e., long-lived iteroparous spawners (Stearns, 1976), and weakfish appear to fit this category very well. First, weakfish exhibit bi-modal spawning activity, exhibiting a pulse in May, decreased activity in late June and early July, and a second pulse in late July (Lowerre-Barbieri et al. 1996). The early weakfish spawning coincides with the expected spring bloom of zooplankton in temperate estuaries, thus risking the increased mortality associated with low temperatures (17[degrees]C-19[degrees]C) for the compensating ad vantage of higher prey availability. The depression in zooplankton abundance that follows in July is accompanied by elevated temperatures (22[degrees]C-25[degrees]C). Prey availability may be lower, but there is an opportunity for rapid growth if sufficient prey is found. Second, weakfish reach sexual maturity rapidly; the onset of sexual maturity can occur after just one year (Merriner, 1976). Third, adult weakfish are long-lived ([less than]10 y) and demonstrate iteroparous spawning of individuals. This is exactly the suite of life-history tactics expected in a species reproducing in an environment with a high risk of recruitment failure for any particular cohort of larvae.
Thanks to D. Breitburg, M. Taylor, P. Gaffney and two anonymous reviewers whose comments helped to improve this manuscript. Support for this work was provided by the International Women's Fishing Association and by Sigma Xi, The Scientific Research Society, as grants to JDA.
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Mean growth-in-weight ([G.sub.w]) of weakfish (Cynoscion) regalis) larvae at each of three prey densities in field enclosures. Early-Stage = growth from 2-9 DPH. Late-Stage = growth from 9-17 DPH. Standard deviation in parentheses. Adapted from Duffy et al. 1996. YEAR, STAGE DENSITY (PREY [L.sup.-1]) [G.sub.w] ([D.sup.-1]) 1990 Early-Stage 10 0.14 (0.08) 100 0.15 (0.01) 1000 0.21 (0.01) Late-Stage 10 0.10 (0.05) 100 0.15 (0.03) 1000 0.19 (0.04) 1994 Late-Stage 10 0.07 (0.03) 100 0.14 (0.03) 1000 0.16 (0.03) Results of regression analyses calculating the relationships between prey density (P) and growth-in-weight ([G.sub.w]) among Early-Stage and LateStage weakfish larvae. LARVAL STAGE SLOPE INTERCEPT N [R.sup.2] p-VALUE Early-Stage 0.015 0.1 6 0.92 0.003 Late-Stage 0.02 0.05 6 0.87 0.01 Model results of estimated duration of the larval stage (days) of a weakfish larva under conditions of Low (10 prey [I.sup.-1]), Mid (100 prey [I.sup.-1]), and High (1000 prey [I.sup.-1]) prey densities. PREY DENSITY EARLY-STAGE LATE-STAGE TOTAL LARVAL (PREY [L.sup.-1]) STAGE DURATION 10 9.2 16.5 25.6 100 7.6 11.1 18.4 1000 6 8.4 14.4 Percent survival at 100 prey [liter.sup.-1] over the range of rates of instantaneous mortality (Z) used in the model. Z ([D.sup.-1]) % SURVIVAL TO END OF THE % SURVIVAL TO THE END OF THE EARLY-STAGE LATE-STAGE 0.1 46.6 15.6 0.2 21.7 2.5 0.3 10.1 0.4 0.4 4.7 0.06 0.5 2.2 0.01 0.6 1 0.001 0.7 0.5 2.5 X [10.sup.-4] 0.8 0.2 4 x [10.sup.-5] 0.9 0.1 6.4 x [10.sup.-6] 1 0.05 1 x [10.sup.-6]
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|Author:||DUFFY-ANDERSON, JANET T.; EPIFANIO, CHARLES E.|
|Publication:||Bulletin of the New Jersey Academy of Science|
|Date:||Mar 22, 1999|
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