Joseph A. Brown

Effects of docosahexaenoic, eicosapentaenoic, and arachidonic acids on the early growth, survival, lipid composition and pigmentation of yellowtail flounder (Limanda ferruginea): a live food enrichment experiment

The role of dietary ratios of docosahexaenoic acid (DHA, 22:6n−3), eicosapentaenoic acid (EPA, 20:5n−3) and arachidonic acid (AA, 20:4n−6) on early growth, survival, lipid composition, and pigmentation of yellowtail flounder was studied. Rotifers were enriched with lipid emulsions containing high DHA (43.3% of total fatty acids), DHA+EPA (37.4% and 14.2%, respectively), DHA+AA (36.0% and 8.9%), or a control emulsion containing only olive oil (no DHA, EPA, or AA). Larvae were fed differently enriched rotifers for 4 weeks post-hatch. At week 4, yellowtail larvae fed the high DHA diet were significantly larger (9.7±0.2 mm, P<0.05) and had higher survival (22.1±0.4%), while larvae fed the control diet were significantly smaller (7.3±0.2 mm, P<0.05) and showed lower survival (5.2±1.9%). Larval lipid class and fatty acid profiles differed significantly among treatments with larvae fed high polyunsaturated fatty acid (PUFA) diets having higher relative amounts of triacylglycerols (18–21% of total lipid) than larvae in the control diet (11%). Larval fatty acids reflected dietary levels of DHA, EPA and AA while larvae fed the control diet had reduced amounts of monounsaturated fatty acids (MUFA) and increased levels of PUFA relative to dietary levels. A strong relationship was observed between the DHA/EPA ratio in the diet and larval size (r2=0.75, P=0.005) and survival (r2=0.86, P=0.001). Following metamorphosis, the incidence of malpigmentation was higher in the DHA+AA diet (92%) than in all other treatments (∼50%). Results suggest that yellowtail larvae require a high level of dietary DHA for maximal growth and survival while diets containing elevated AA exert negative effects on larval pigmentation.

Foraging, growth and survival of Atlantic cod larvae reared in different light intensities and photoperiods

Abstract Studies have shown that most marine fish larvae are visual feeders and their feeding incidence increases with light intensity. Although laboratory studies on photoperiod showed variable results, evidence from the field suggest that larvae exposed to a continuous light may grow better than larvae exposed to reduced photoperiods. We set up experiments to investigate the foraging, growth and survival of Atlantic cod larvae ( Gadus morhua ; Grand Banks origin) at different light intensities and photoperiods. Behavioural observations were also carried out in an attempt to explain any differences in the performance of cod larvae under varying light intensities. Larval cod were reared at four light intensities (300, 600, 1200 and 2400 lx or 4.93, 10.96, 19.89 and 37.32 μE m −2 , respectively) and three photoperiods (24 L:0 D, 18 L:06 D and 12 L:12 D) from hatching to metamorphosis (42 days post-hatch; dph). Cod larvae grew and survived better in higher light intensity (2400 lx) and 24 L:0 D photoperiod. Examination of the foraging indicated that cod larvae reared in higher light intensity captured prey more efficiently than larvae reared in low light. Results also showed that the mortality rates of larval cod from 2400 lx and 24 h photoperiod were significantly lower than the larvae from other light intensity and photoperiod treatments until 28 dph. This indicates that photoperiod and light levels could be reduced beyond 28 dph.

Little left in the tank: metabolic scaling in marine teleosts and its implications for aerobic scope.

Fish larvae are the worlds smallest vertebrates, and their high rates of mortality may be partially owing to a very limited aerobic scope. Unfortunately, however, no complete empirical dataset exists on the relationship between minimal and maximal metabolism (and thus aerobic scope) for any fish species throughout ontogeny, and thus such an association is hard to delineate. We measured standard and maximal metabolism in three marine fish species over their entire life history, and show that while aerobic scope depends greatly on body size and developmental trajectory, it is extremely small during the early life stages (factorial aerobic scope≤1.5). Our findings strongly suggest that limited scope for aerobic activity early in life is likely to constrain physiological function and ultimately impact behaviour and possibly survival. Furthermore, our results have important implications for ecological models that incorporate metabolic scaling, and provide additional evidence against the existence of ‘universal’ scaling exponents.

Genetic variation in life-history reaction norms in a marine fish.

Neither the scale of adaptive variation nor the genetic basis for differential population responses to the environment is known for broadcast-spawning marine fishes. Using a common-garden experimental protocol, we document how larval growth, survival and their norms of reaction differ genetically among four populations of Atlantic cod (Gadus morhua). These traits, and their plastic responses to food and temperature, differed across spatial scales at which microsatellite DNA failed to detect population structure. Divergent survival reaction norms indicate that warm-water populations are more sensitive to changes in food, whereas cold-water populations are more sensitive to changes in temperature. Our results suggest that neither the direction nor the magnitude of demographic responses to environmental change need be the same among populations. Adaptive phenotypic plasticity, previously undocumented in marine fishes, can significantly influence the probability of recovery and persistence of collapsed populations by affecting their ability to respond to natural and anthropogenic environmental change.

Larviculture of Atlantic cod (Gadus morhua): progress, protocols and problems

Interest in the intensive culture of Atlantic cod has increased dramatically due to a reduced supply from the wild fishery, high market price and relative suitability of cod for culture. In spite of its many positive attributes for domestication, intensive culture has only been successful during the past 6 years. Large commercial efforts are currently underway in Norway, Scotland, Newfoundland, Canada and New Hampshire, USA. Cod are a geographically widespread species and because of this certain population differences in response to environmental factors, such as light and prey density exist. These differences must be taken into account when developing protocols for commercial production. Research in Newfoundland has focussed on determining optimal prey densities and light levels to use during larviculture. Using a prey density of 4000 prey/l and light intensities above 2000 lx, survival rates of over 40% from start-feeding to metamorphosis have been achieved. A comparison of production protocols used in Newfoundland to efforts in Norway is discussed.

Substrate selection by juvenile Atlantic cod (Gadus morhua): effects of predation risk

Although predator avoidance has been proposed as one possible factor influencing the distribution of fish among substrate types, no study has addressed this question directly. Groups of juvenile Atlantic cod were offered a choice between pairs of the following three substrates: sand, gravel-pebble and cobble. Their distribution on these substrates was compared prior to, during and following exposure to a predator (i.e. a larger conspecific). With no apparent risk of predation, juvenile cod preferred sand or gravel-pebble. When cobble was present, juveniles hid in the interstitial spaces of this substrate in the presence of a predator. With no cobble present, juveniles showed no preference between sand and gravel-pebble, and did not seek refuge from predation in association with these substrates. Following exposure to a predator (i.e. 2.5 h later) larger juvenile cod again showed a preference for the finer-grained substrates, but smaller individuals continued to associated with the cobble. The presence of cobble resulted in fewer juveniles being captured and a significant increase in the latency until the first juvenile was captured by the predator. Results are discussed with respect to the effects of predation on the distribution and survival of fishes among substrate types.

Feeding frequency affects food consumption, feeding pattern and growth of juvenile yellowtail flounder (Limanda ferruginea).

Feeding experiments were carried out on juvenile yellowtail flounder (Limanda ferruginea) (6.8±0.2 g), a candidate for cold-water aquaculture in eastern North America. At about 7 °C, feeding frequency was shown to have a significant (P<0.05) effect on food consumption and growth, with fish fed to apparent satiation in two or four daily meals consuming more food and growing better than fish fed less often (once daily and two meals every other day). Fish fed two meals every other day ate significantly more (P<0.05) in the morning (8.4±0.4 mg) than in the afternoon (6.0±0.2 mg), whereas there were no differences in meal sizes between feedings for other groups. Behavioural observations revealed that fish fed fewer meals per day ingested more pellets per feeding (1.1±0.1 vs. 0.7±0.1 pellet/min), showed more activity and foraged more often throughout the day than fish fed four times daily or twice daily (3.5±0.3 vs. 2.1±0.2 behaviour/min). Low incidences of aggression indicated that yellowtail flounder probably do not form feeding hierarchies; however fish fed twice daily showed a significant increase in the coefficient of variation (CV) for body weight over the course of the experiment (P<0.05). Fish fed twice daily gained significantly more weight (P<0.05) and had the lowest FCR (0.89), thus it is recommended that fish at this stage of grow-out be fed twice per day.

Foraging, growth and survival of Atlantic cod larvae reared in different prey concentrations

Abstract Laboratory studies have shown that suboptimal prey concentrations during the first feeding usually result in slower growth and higher mortality. We investigated the growth, survival and foraging behaviour of Atlantic cod (Gadus morhua) larvae in different prey concentrations under laboratory conditions. Larvae were reared from hatching to metamorphosis (6 weeks post-hatch) on rotifers and/or Artemia nauplii at 250, 500, 1000, 2000, 4000, 8000 and 16,000 prey l−1. Once a week, standard length and myotome height were recorded and mortalities were monitored from day 15. Observations on feeding behaviour were recorded twice a week. Larvae reared at 4000–16,000 prey l−1 were significantly larger and had significantly higher condition indices than larvae from lower prey concentration (≤2000). Initially, specific growth rates (SGR) were significantly higher in larvae reared at higher prey concentrations (≥4000), but from 3 weeks post-hatch, no difference was found among treatments in SGR. Instantaneous mortality rates (Z) decreased in all treatments as the larvae grew. Survival was significantly lower for larvae reared at lower prey concentrations until week 3. Larvae reared at 250 and 500 prey l−1 did not survive beyond days 11 and 24, respectively. Survival of larvae was significantly higher at higher prey concentrations at weeks 2 and 6 post-hatch. Although survival of cod larvae at week 2 was similar among the three highest prey concentrations; survival at the end of the experiment (week 6) was higher, but not significantly, in the 4000 prey l−1 treatment than the other two treatments (8000 and 16,000 prey l−1). Swimming duration was significantly higher among larvae reared at lower prey concentrations. Larvae reared at higher prey concentrations (≥4000) showed higher orientation and captured success rates but lower attack rates than larvae reared at lower prey concentrations. Our results indicate that for intensive rearing of cod larvae, higher prey concentrations are required to sustain reasonable growth and survival than that have been reported for mesocosm studies.

Habitat use by juvenile Atlantic cod (Gadus morhua) in the presence of an actively foraging and non-foraging predator

Experiments were conducted in the autumn and winter of 1992/93 to examine habitat use by juvenile (age 0+) Atlantic cod, Gadus morhua L., before, during and following exposure to a passive or actively foraging predator (age 3+ cod). Experiments presented groups of juvenile cod (n=5 fish/group) with one of two combinations of three substrates; (1) gravel, sand, and a patch of artificial kelp (“kelp”), or (2) cobble, sand, and kelp. Cobble is known to provide juvenile cod with a refuge from predation. Kelp was used to test the hypothesis that juvenile cod associate with fleshy macroalgae in nature because of the safety it provides from predators. There was little difference in habitat use by juvenile cod before, during or following exposure to a passive predator. Under these conditions, juvenile cod appeared to prefer finer grained mineral substrates and avoided the kelp. The extent of the juvenile response to a passive predator was to avoid the predators location in the experimental tank. In contrast, juvenile cod showed a significant shift in habitat use when exposed to an actively foraging predator, hiding in cobble or, when cobble was not available, in kelp. Use of both these habitats resulted in a significant reduction in predation risk to the juvenile cod. Our results suggest that: (1) an association with kelp provides safety from predation to juvenile cod, and (2) juvenile cod are capable for assessing the risk a predator represents and adjust their response accordingly.

Kin discrimination in salmonids

SummaryThe data presented here suggest that significant selection pressures towards kin discrimination behaviour patterns result from kin-biased territorial defence behaviour patterns. Salmonids employ a phenotype matching recognition mechanism allowing individuals to discriminate unfamiliar kin. Kin discrimination abilities allow individuals to reduce the levels of aggression associated with territorial defence towards related conspecifics and to defend smaller territories near kin versus non-kin. This kin-biased territorial defence behaviour is observed in at least one species under a wide range of territorial quality conditions. Within kin groups, subordinate individuals obtain a greater number of foraging attempts, resulting in kin-biased foraging within the social group. As a result of this kin-bias, individuals within kin groups show significantly higher mean weight increases (increased direct fitness benefits) and reduced variance in these increases (increased indirect benefits). Since all individuals within the kin groups obtained higher, less variable weight increases, we can argue that individuals are increasing their inclusive fitness as a result of these kin-biased behaviour patterns.Based on these results, and on what is known about the life history of a variety of salmonid and non-salmonid species, we can formulate a number of testable predictions. By testing these predictions, we may be better able to understand both the proximate and ultimate causation of kin discrimination abilities in a variety of fishes.