James H. Cowan

BEHAVIOR AND RECRUITMENT SUCCESS IN FISH LARVAE: REPEATABILITY AND COVARIATION OF SURVIVAL SKILLS

Survival of larvae of highly fecund fishes is often considered to be random, although some investigators have hypothesized that survivors are exceptional individuals. If behavior is important to survival, individuals that perform both well and repeatably will be favored. We conducted experiments on red drum larvae (Sciaenops ocellatus, mean length = 7.7 mm) to assess individual variability in skills related to foraging and predator evasion. We measured routine swimming speed and, for responses to two startle stimuli, response score, latency, distance, duration, and speed. We tested 100 larvae five times each. A large proportion of larvae exhibited repeatable performance for 10 of the performance variables. However, repeatability (or performance level) in one skill was not correlated with repeatability (or level) in an independent skill, except for a positive correlation between acoustic and visual response scores. This result suggested that the level of attentiveness to predators varies among individuals....

Analysis of an Estuarine Striped Bass Population: Effects of Environmental Conditions During Early Life

Estuarine fish populations are exposed to a variety of environmental conditions that cause both short-term variability and long-term trends in abundance. We analyzed an extensive data set for striped bass (Morone saxatilis) in the San Francisco Estuary to refine our understanding of how environmental variability influences recruitment. We examined the effects of environmental variability during early life stages on subsequent recruitment (age 3 yr), and the degree to which conditions in early life may have contributed to a long-term decline in abundance of adult striped bass in the San Francisco Estuary. Survival from egg to young-of-the-year varied strongly with freshwater flow; this effect apparently occurred within the first week or two of life, a time period that encompasses transport of eggs and larvae from the rivers to rearing areas and the onset of feeding. The rate of freshwater flow to pumping facilities that export freshwater from the system had small or sporadic effects on survival during the first month or two of life. Although many young striped bass between ages 2 and 8 mo were entrained in export pumping facilities, the resulting high mortality was unrelated to total mortality rates determined from field data on young striped bass. This lack of effect was apparently due to strong density-dependent mortality occurring between ages 1 mo and 3 yr (Kimmerer et al. 2000). The available data do not support previously suggested relationships between recruitment and freshwater flow during early life, or between gross estimates of pesticide input and survival of early life stages. We used a simple life-cycle model to show that various combined factors could have led to a decline in adult abundance, particularly a large and increasing adult mortality, but that events early in life probably did not contribute substantially to the decline. These results demonstrate that several decades of monitoring data from numerous life stages are needed to distinguish among alternative hypotheses about environmental influences on populations of estuarine fish.

Enclosure experiments on survival and growth of black drum eggs and larvae in lower Chesapeake bay

Experiments in 2.2 m3, in situ mesocosm enclosures indicate that black drum, Pogonias cromis, eggs and larvae potentially can survive in the lower Chesapeake Bay at ambient microzooplankton prey levels (≈200 prey 1−1) in the absence of predators. In growth experiments, larva mean growth rates to 10 d posthatch were similar (0.17 mm d−1 and 0.18 mm d−1) when fed at prey levels of 50 prey 1−1 and 200 prey 1−1. Individual growth rates, however, were more variable at 50 prey 1−1. Mortality rates also were comparable in 50 (27% d−1) and 200 (23% d−1) prey 1−1 enclosures. In a second experiment, the predation potentials of the hydromedusa Nemopsis bachei and the lobate ctenophore Mnemiopsis leidyi were estimated in relation to initial black drum egg prey density, presence of alternative <1 mm zooplankton prey, and estimated daily abundance of the jellyfish on the black drum spawning grounds. Mortality rates per medusa and ctenophore were similar (0.02–0.03 d−1), were not affected by presence of alternative prey, and were directly related to initial egg density. Results suggest that the gelatinous predators, especially the hydromedusa, could have cleared a high (≈38%) but variable fraction of the water column daily of fish eggs and yolk-sac larvae during the black drum spawning season. We hypothesize that the poor or episodic recruitment success of black drum in Chesapeake Bay results from a short spawning season that often coincides with abundance peaks of gelatinous predators and that predation on eggs and yolk-sac larvae may control recruitment.

Size-based foraging success and vulnerability to predation: selection of survivors in individual-based models of larval fish populations

Because small changes in larval fish cohort-specific growth and mortality rates can generate order-of-magnitude or greater differences in annual recruitments (Shepherd and Cushing, 1980; Houde, 1987, 1989; Davis et al., 1991), it has been surmised that increased larval size improves an individual’s probability of survival. It is reasoned that increased larval size should improve foraging ability, further resulting in increased growth rate, shorter stage duration, and/or reduced vulnerability to predation. This supposition implies that large individuals are more likely to survive to metamorphosis (Peterson and Wroblewski, 1984; McGurk, 1986; Anderson, 1988; Miller et al., 1988; Beyer, 1989; Pepin, 1989a,b, 1991; Cowan and Houde, 1992; Rice et al., 1993a,b). However, the cumulative effects of individual, size-based interactions between foraging success and predation vulnerability have seldom been evaluated at the population (cohort) level.

Predicting fish population dynamics: compensation and the importance of site-specific considerations

Abstract The magnitude of the compensatory response inherent to fish population seems to be questioned with each new generation of scientists and regulators. The major reason for this is the demand by some that compensation be completely understood at the process level before compensation can be factored into population predictions. We first discuss several lines of evidence for the existence of compensation, and the reasons why compensation is so difficult to measure at the process level. We then offer life history theory as a general framework for improving our understanding of the likely magnitude of compensation in fish population dynamics. We use detailed, individual-based models and field data for yellow perch-walleye, striped bass, and a lake community to illustrate the complexity of compensation, and to show that predictions for a particular population require site-specific information. Analyses show that the life stage and magnitude of density-dependence can vary for different populations of the same species, and that population responses can depend on the arrangement of the food web. We must stop reinventing the density-dependence wheel; too many resources (both natural and monetary) are at stake. We suggest using three approaches for addressing the compensation issue: (1) life history theory for providing a sound basis for qualitatively understanding fish population dynamics and the magnitude of compensation; (2) age- and stage-structured matrix models for screening-level analyses to identify populations that may be at unacceptable risks of decline and therefore require additional analyses; and (3) individual-based models for better understanding the complexities of compensation at the process level and for impact assessment of at-risk, well-studied populations.

Oil and Gas Platforms in the Gulf of Mexico: Their Relationship to Fish and Fisheries

There are over 2300 standing oil and gas platforms in the northern Gulf of Mexico (GOM). It has been argued that platforms provide reef-like habitat that increases the growth and survival rates of fishes by increasing prey availability and affording shelter for protection from predators, provide additional spawning substrate, and by acting as a visual attractant for organisms not otherwise dependent upon hard bottom. Platforms differ from most natural habitats, and from traditional artificial reefs, in that their vertical profile extends upward through the water column into the photic zone and the sea surface. Increased habitat quality on, or immediately around, oil and gas platforms are thought to be derived from increased in situ food production associated with encrustation by fouling organisms. In this chapter, we address the issue of how to evaluate the role of artificial reefs by first establishing levels of evaluation for individual fish species found on oil and gas platforms in the GOM. The levels of evaluation relate to the amount and adequacy of the available information, which was populated with an extensive literature and data search. Three levels of assessment are established, analogous to the levels of analysis established National Oceanographic and Atmospheric Administration (NOAA) Fisheries for identification of Essential Fish Habitat. More than 1300 documents, including reports, stock assessments, other gray literature, and papers published in the primary literature, were used to complete this chapter. When available, published literature was the preferred source of information.