George A. Rose

Changes in proximate composition and somatic energy content for Pacific sand lance (Ammodytes hexapterus) from Kachemak Bay, Alaska relative to maturity and season

Mean dry-weight energy values of adult Pacific sand lance (Ammodytes hexapterus) peaked in spring and early summer (20.91 kJg−1 for males, 21.08 kJg−1 for females), then declined by about 25% during late summer and fall (15.91 kJg−1 for males, 15.74 kJg−1 for females). Late summer declines in energy density paralleled gonadal development. Gender differences in energy density (males<females) were only apparent from August to October. Adult sand lance spawn in October, entering the winter with close to their minimum whole body energy content. Juvenile sand lance exhibited a relatively constant protein to lipid ratio until they reached 80 mm fork length. Thereafter, relative proportions of protein remained constant while lipid proportions increased significantly. Dry weight energy densities of juveniles increased from a minimum 16.67 kJg−1 to a maximum of 19.68 kJg−1, and (per g) are higher than adults in late summer. The seasonal food value of adult sand lance to predators varies markedly, but maximum energetic value coincides with important feeding periods for marine mammals, fish, and seabirds.

Cross-shoal variability in the feeding habits of migrating Atlantic cod (Gadus morhua)

Prey intake and selection were related to within-shoal position for Atlantic cod (Gadus morhua) engaged in annual migration across the Newfoundland shelf in the northwest Atlantic. Comparisons made among fish occupying five regions from the front to rear of a large (>10 km across) migrating shoal indicated that leading fish, or scouts, were larger, ate more food by weight, and had a more varied diet than did fish at other positions. Also, scouts consumed more preferred prey types (fish and pelagic invertebrates) than did fish at other positions. In contrast, trailing fish consumed few fish prey but a larger proportion of benthic invertebrates. Our results are the first to document systematic heterogeneous feeding success among members of a free-ranging and migrating fish shoal in the open ocean.

Sex differences in cod residency on a spawning ground

Abstract Telemetric methods were employed to explore behaviour differences between male (n=21) and female (n=27) Atlantic cod (Gadus morhua) tagged and released in April 1998 on a spawning ground in coastal Newfoundland. Their distribution was monitored over a grid of listening stations for three consecutive spawning seasons (1998–2000). Of the 48 tagged fish, 32 were relocated acoustically and 15 caught in the fishery (including seven previously relocated) during this study. Only eight were never relocated. On average, males stayed on the spawning ground for at least 10 days, and females for 19 days. The rate of departure from the spawning ground varied between years. There was no effect of body length on the timing of departure from the spawning ground, for either males or females. Males departed from the grounds earlier than females. The relocation rates for males were greater than for females in all years. In any given survey of the spawning ground, resident males were more likely to be relocated, suggesting that females move in and out of male-dominated spawning aggregations. Mobile and intercept fisheries on cod spawning grounds could disrupt the movements of spawning females or lead to sex-biased harvest rates.

Monitoring coastal northern cod: towards an optimal survey of Smith Sound, Newfoundland

The extant, coastal northern cod (Gadus morhua) have over-wintered and spawned in Smith Sound, Newfoundland, since 1995, and acoustic surveys have been conducted in several seasons since then. Cod move into the Sound in late fall, over-winter in a dense, size- and age-structured aggregation, spawn between late March and early June and then disperse into and beyond Trinity Bay during summer to feed. The optimal survey time for biomass estimation is January-February, when the waters are ice-free and the cod are in mono-specific, relatively stationary, and well-defined aggregations with the highest densities and are typically clear of “bottom returns”. Biomass surveys have been conducted in mid-January since 1999. An error analysis indicated the main sources of uncertainty to be density variability and target strength (TS). Repeated quasi-synoptic (10 h) surveys were the optimal means of producing an estimate of uncertainty about population size. Some vertical movement led to night-time surveys consistently having higher estimates than day-time surveys by approximately 15%. Detectability ranged from 73 to 86% and deadzone-corrected, acoustic measures did not differ from swept-area densities found by bottom trawling. Biomass scaling by TS used length-dependant dB/kg to reduce the size-sampling error. Overall, population biomass doubled in approximately 7 years, consistent with a rate of increase around 0.2, largely through recruitment. The surveys are internally consistent and indicate instantaneous rates of mortality among year classes of 0.3 to >2 (very high on older fish) and provide a method for monitoring the annual biomass (cv<40%).

A review of problems and new directions in the application of fisheries acoustics on the Canadian East Coast

Abstract Applications of fisheries acoustics on the Canadian East Coast have progressed from qualitative echogram interpretations of fish distribution to quantitative multispecies biomass mensurations over the past 50 years. These developments have depended on innovations in electronic, sonar, and computer technology. Instrumentation is now quite sophisticated — but our abilities to assess its outputs in biological terms are not. Continued progress requires better accuracy and precision through improved: (1) calibration of system voltage output over time and between systems; (2) conversion of voltage output to biological information (distribution, abundance, biomass, size, and species); (3) measurement and resolution of fish near boundaries (surface and bottom); (4) sampling designs and analytical techniques. New directions in signal processing and interpretation include improved data display techniques, higher resolution numeric and image analyses, and the use of naturally occurring biologically, physically, and ecologically determined scales of fish aggregation as the spatial and temporal basis for collection, synthesis and analysis of acoustic data. The lack of trained personnel impedes progress.

Out-migration of tagged fishes from marine reef National Parks to fisheries in coastal Kenya

We evaluated movements of 25 species of coral reef fishes from Malindi and Watamu Marine National Parks (created 1968) in coastal Kenya from February 2001 to March 2002. Only three species, the commercially important whitespotted rabbitfish, Siganus sutor, the sky emperor (SEM), Lethrinus mahsena and the trumpet emperor, L. miniatus, exhibited consistent movements from the parks. At Malindi Park, more fishes were recaptured by fishermen off a fringing reef than off a patch reef. The rabbitfish had a higher monthly spillover rate from the fringing reef than from the patch reef. In contrast, the SEM had low monthly spillover rates from both reefs. The rabbitfish moved greater distances off the fringing reef than off the patch reef. At Watamu Park, the SEM, L. miniatus and the gold-spotted sweetlips, Gaterin flavomaculatus, had equal monthly spillover rates. In contrast, the rabbitfish had a lower monthly rate. The emperors showed no difference in net distance moved from the park boundary, however, L. miniatus traveled significantly longer distances than did the SEM. Distances between release and capture sites were either random (SEM), increasing (L. miniatus), or decreasing (rabbitfish) with respect to time at liberty.

Growth and Abundance of Pacific Sand Lance, Ammodytes hexapterus, under differing Oceanographic Regimes

Dramatic changes in seabird and marine mammal stocks in the Gulf of Alaska have been linked to shifts in abundance and composition of forage fish stocks over the past 20 years. The relative value (e.g., size and condition of individual fish, abundance) of specific forage fish stocks to predators under temporally changing oceanographic regimes is also expected to vary. We inferred potential temporal responses in abundance, growth, and age structure of a key forage fish, sand lance, by studying across spatially different oceanographic regimes. Marked meso-scale differences in abundance, growth, and mortality existed in conjunction with these differing regimes. Growth rate within stocks (between years) was positively correlated with temperature. However, this relationship did not exist among stocks (locations) and differing growth rates were better correlated to marine productivity. Sand lance were least abundant and grew slowest at the warmest site (Chisik Island), an area of limited habitat and low food abundance. Abundance and growth of juvenile sand lance was highest at the coolest site (Barren Islands), an area of highly productive upwelled waters. Sand lance at two sites located oceanographically between the Barren Islands and Chisik Island (inner- and outer-Kachemak Bay) displayed correspondingly intermediate abundance and growth. Resident predators at these sites are presented with markedly different numbers and quality of this key prey species. Our results suggest that at the decadal scale, Gulf of Alaska forage fish such as sand lance are probably more profoundly affected by changes in abundance and quality of their planktonic food, than by temperature alone.

Long-distance movements of coral reef fishes

Most coral reef fishes are thought to be highly sedentary with movements limited to a few kilometers for even the most mobile species (Holland et al. 1996; Zeller 1997). Long distance (>200 km) movements to spawning aggregations have been reported for Epinephelus striatus (Nassau grouper) often with return to home reefs. Other reports on large spatial-scale movements in tropical fishes have been limited to pelagic and non-typical reef species like the blue runner, Caranx fuses (155 km) (Beaumariage 1964) and the blue trevally, Caranx melampygus (72 km) (Holland et al. 1996). Movements are demanding in energy requirements (Bernstein et al. 1991) and are not without functional role. Among coral reef fishes, local movements have been associated with feeding, spawning, and ontogenetic shifts in habitat requirements (Sale 2002). Additionally, movements may ensure connectivity between stocks. However, a major unanswered question in marine ecology is the degree of connectedness between populations (Robert et al. 2000). In coral reef environments, the open nature of reproduction in most fishes has made pelagic larval dispersal the dominant linkage between reef fish populations (Cowen et al. 2000). However, active adult dispersal between source and sink populations (Crowder et al. 2000) may complement larval dispersal in maintaining connectivity between populations and is thought to be a more stable mechanism evolutionarily (Holt 1996). In this paper we document long-distance movements in three species of coral reef fishes.

Feeding of capelin (Mallotus villosus) in Newfoundland waters

Abstract The diet of capelin (Mallotus villosus Müller) from six areas off the Newfoundland and Labrador coast was compared over three seasons (January, May–June, August–September) in 1999. A total of 1247 stomachs were examined. Of these, 837 (67 %) contained food. The proportion of empty stomachs was higher in winter (55 %) than in spring (28 %) or autumn (20 %). Copepods were the major prey over all areas and seasons, occurring in 90 % of non-empty stomachs. Hyperiid amphipods, euphausiids, larvaceans and chaetognaths were also important, occurring in 30 %, 11 %, 9% and 7% of non-empty stomachs respectively. The importance of these other prey groups increased with increasing capelin size. Larger capelin contained larger prey. There were also spatial and temporal differences in diet. Capelin from Placentia Bay, southeastern Newfoundland, consumed smaller copepods and a higher proportion of amphipods than capelin from other areas. Diet composition, particularly the incidence of lipid-rich Calanus species, may influence capelin growth.

Dietary niche partitioning in sympatric gadid species in coastal Newfoundland: evidence from stomachs and C-N isotopes

The feeding habits of co-occurring gadid species Atlantic cod (Gadus morhua) and Greenland cod (Gadus ogac) in coastal Newfoundland waters, examined using stable isotope (δ13C and δ15N) and stomach content analysis, indicated little dietary niche overlap and interspecific competition for food resources despite similar trophic levels. Both species consumed a variety of invertebrates and fish but showed a preference for different prey items. Polychaetes, fish and small crustaceans dominated G. ogac stomach contents while small crustaceans, in particular hyperiid amphipods and fish, dominated those of G. morhua. In general, G. morhua consumed more pelagic prey and had a significantly more pelagic (more negative) δ13C signature while G. ogac consumed primarily benthic prey and had a more benthic (more positive) δ13C signature. δ15N levels were similar in these species suggesting similar trophic positions, with levels increasing with fish length in both species. Dietary overlap was not significant in both stomach and stable isotope analyses. We conclude that interspecific competition for food is low between G. ogac and G. morhua and is unlikely to be a factor in the slow rebuilding of Atlantic cod in this region.