Spatial and seasonal relationships between Pacific harbor seals (Phoca vitulina richardii) and their prey, at multiple scales

Knowing where pinnipeds forage is vital to managing and protecting their populations, and for assessing potential interactions with fisheries. We assessed the spatial relationship between the seasonal distribution of Pacific harbor seals (Phoca vitulina richardii) outfitted with satellite transmitters and the seasonal distributions of potential harbor seal prey species in San Francisco Bay, California. Pearson’s correlation coefficients were calculated between the number of harbor seal locations in an area of the San Francisco Bay and the abundance of specific prey species in the same area. The influence of scale on the analyses was assessed by varying the scale of analysis from 1 to 10 km. There was consistency in the prey species targeted by harbor seals year-round, although there were seasonal differences between the most important prey species. The highest correlations between harbor seals and their prey were found for seasonally abundant benthic species, located within about 10 km of the primary haul-out site. Probable foraging habitat for harbor seals was identified, based on areas with high abundances of prey species that were strongly correlated with harbor seal distribution. With comparable local data inputs, this approach has potential application to pinniped management in other areas, and to decisions about the location of marine reserves designed to protect these species.

Age- and length-at-maturity of female arrowtooth flounder (Atheresthes stomias) in the Gulf of Alaska

Arrowtooth flounder (Atheresthes stomias) has had the highest abundance of any groundfish species in the Gulf of Alaska since the 1970s (Matarese et al., 2003; Turnock et al., 2005; Blood et al., 2007); however, commercial catches have been restricted because Pacific halibut (Hippoglossus stenolepis) are caught as bycatch in the fishery. Arrowtooth flounder plays a key role in the ecosystem because it is a dominant organism within the food web, both as an apex predator of fish and invertebrates, as well as an important prey for walleye pollock (Theragra chalcogramma; Aydin et al., 2002). Walleye pollock is the dominant groundfish in the Bering Sea, a principal groundfish in the Gulf of Alaska, and the primary prey for marine mammals. The distribution of arrowtooth flounder extends from Cape Navarin and the eastern Sea of Okhotsk in Russia, across the Bering Sea, Aleutian Islands, Gulf of Alaska, and south to the coast of central California (Shuntov, 1964; Britt and Martin, 2001; Chetvergov, 2001; Weinberg et al., 2002; Zenger, 2004). Because of the importance of arrowtooth flounder in the marine ecosystem of A laska, a maturity study of this species was undertaken to determine age-at-maturity, which is essential for age-based stock management models. Before these results, management has had to rely upon a length-at-maturity-based estimate (Zimmermann, 1997) to manage stocks in the Gulf of Alaska (GOA), Bering Sea, and Aleutian Islands. The central GOA was selected as the location for this maturity study Age- and length-at-maturity of female arrowtooth flounder (Atheresthes stomias) in the Gulf of Alaska because it contains approximately 70% of the total Gulf of Alaska arrowtooth flounder biomass (1.9×106 t, age 3 and older)— the highest percentage in the world (Shuntov, 1964; Britt and Martin, 2001; Weinberg et al., 2002; Wilderbuer and Nichol, 2006).

The trophic dynamics of summer flounder (Paralichthys dentatus) in Chesapeake Bay

Data on the trophic dynamics of fishes are needed for management of ecosystems such as Chesapeake Bay. Summer flounder (Paralichthys dentatus) are an abundant seasonal resident of the bay and have the potential to impact foodweb dynamics. Analyses of diet data for late juvenile and adult summer flounder collected from 2002−2006 in Chesapeake Bay were conducted to characterize the role of this flatfish in this estuary and to contribute to our understanding of summer flounder trophic dynamics throughout its range. Despite the diversity of prey, nearly half of the diet comprised mysid shrimp (Neomysis spp.) and bay anchovy (Anchoa mitchilli). Ontogenetic differences in diet and an increase in diet diversity with increasing fish size were documented. Temporal (inter- and intra-annual) changes were also detected, as well as trends in diet reflecting peaks in abundance and diversity of prey. The preponderance of fishes in the diet of summer flounder indicates that this species is an important piscivorous predator in Chesapeake Bay.

Diets of and trophic relationships among dominant marine nekton within the northern California Current ecosystem

In this study we analyzed the diets of 26 nekton species collected from two years (2000 and 2002) off Oregon and northern California to describe dominant nekton trophic groups of the northern California Current (NCC) pelagic ecosystem. We also examined interannual variation in the diets of three nekton species. Cluster analysis of predator diets resulted in nekton trophic groups based on the consumption of copepods, euphausiids, brachyuran larvae, larval juvenile fishes, and adult nekton. However, many fish within trophic groups consumed prey from multiple trophic levels—euphausiids being the most widely consumed. Comparison of diets between years showed that most variation occurred with changes in the contribution of euphausiids and brachyuran larvae to nekton diets. The importance of euphausiids and other crustacean prey to nekton indicates that omnivory is an important characteristic of the NCC food web; however it may change during periods of lower or higher upwelling and ecosystem production.

Decline in condition of northern bluefin tuna (Thunnus thynnus) in the Gulf of Maine

The northern bluefin tuna (Thunnus thynnus) is a highly mobile apex predator in the Gulf of Maine. Despite current stock assessments that indicate historically high abundance of its main prey, Atlantic herring (Clupea harengus), commercial fishermen have observed declines in the somatic condition of northern bluefin tuna during the last decade. We examined this claim by reviewing detailed logbooks of northern bluefin tuna condition from a local fishermen’s cooperative and applying multinomial regression, a robust tool for exploring how a categorical variable may be related to other variables of interest. The data set contained >3082 observations of condition (fat and oil content and fish shape) from fish landed between 1991 and 2004. Energy from stored lipids is used for migration and reproduction; therefore a reduction in energy acquisition on bluefin tuna feeding grounds could diminish allocations to growth and gamete production and have detrimental consequences for rebuilding the western Atlantic population. A decline in northern bluefin tuna somatic condition could indicate substantial changes in the bottom-up transfer of energy in the Gulf of Maine, shifts in their reproductive or migratory patterns, impacts of fishing pressure, or synergistic effects from multiple causes.

Geographical differences in the feeding patterns of red rockfish (Sebastes capensis) along South American coasts

Feeding habits and feeding strategy of red rockfish (Sebastes capensis) were studied from fish captured along most of the range of this species in coastal waters of South America. Stomach contents of 613 individuals, collected during 2003, were analyzed. Fish were obtained from six locations along the Chilean (23°S to 46°S) and Argentinian (43°S) coasts. The main prey items were Mysidacea (75.06% IRI), Osteichthyes (6.29% IRI),and Rhynchocinetes typus (6.03% IRI). Predator sex and size did not significantly affect the diet, but significant differences were found between locations. Four geographical areas, discriminated by prey occurrence and frequencies, were determined: three on the Pacific coast and one on the Atlantic coast. These areas correspond roughly with biogeographic zones described for the Chilean and southern Argentinian coasts. The feeding strategy index (FSI) indicated a specialized feeding strategy for S. capensis for most of its range. However, the FSI does not include the behaviour of a predator, and the FSI must be interpreted carefully for fishes like S. capensis that are passive ambush feeders. The abundance and availability of different prey may explain both the geographic differences in dietary composition and the specialized feeding strategy of S. capensis.

Factors influencing cannibalism and abundance of walleye pollock (Theragra chalcogramma) on the eastern Bering Sea shelf, 1982–20

Cannibalism is thought to be an inf luential top-down process affecting walleye pollock (Theragra chalcogramma) recruitment in the eastern Bering Sea (EBS). In summer, many age-1 pollock occupy the same depths as those of adult walleye pollock, making them vulnerable to cannibalism. We examine factors that inf luence the occurrence and amount of cannibalism, as well as the abundance and co-occurrence of predator and prey walleye pollock. Large walleye pollock were generally found in deeper waters and avoided cold temperatures; whereas, age-1 walleye pollock were found in broader bottom depth and temperature ranges. The occurrence of cannibalism was highest in the area where predator and prey walleye pollock co-occurred and the amount of cannibalism was highest on the middle and outer EBS shelf. Both the occurrence and amount of cannibalism were influenced by location, bottom temperature and bottom depth, and the abundance of prey walleye pollock. The abundance of both large and small walleye pollock decreased during the 1982–2006 survey period in the EBS and, hence, the occurrence and amount of cannibalism also decreased. The occurrence and amount of cannibalism observed in the diet samples from the summer survey were good indicators of year class strength, as estimated by the stock assessment model. There was more cannibalism of age-1 walleye pollock when predicted recruit abundance was highest, indicating that summer cannibalism on age-1 walleye pollock, a top-down process, does not control walleye pollock recruitment in the EBS.

Consumption to biomass (Q/B) ratio and estimates of Q/B-predictor parameters for Caribbean fishes

Estimates of the Q/B ratio and parameters of equations to 'predict' Q/B values for 116 fish stocks in the Gulf of Salamanca, Colombia are presented. A compilation of these estimates available for Caribbean Sea fishes (264 stocks) is also provided for comparison purposes. General trends in the value of Q/B resulting from differences in the equation and parameter values used are briefly discussed.

Use of enriched live prey in promoting growth and maturation of tiger shrimp (Penaeus monodon)

This study was undertaken to determine the effect of nutritional management of broodstock of Penaeus monodon on growth and maturation. Test specimens were obtained from a grow-out pond before attainment of maturity and were reared in hatchery tanks. Four types of dietary treatments (M1–M4) were given to separate batches that were run in duplicate. Feeding trials continued for five months. A diet with live bloodworm, bioencapsulated to contain tricalcic phosphate as its major component, was found to be the most efficient. Specimens of this particular batch assimilated food more efficiently, grew at a faster rate and attained maturity earlier than other groups. Bloodworm provided the lipid fractions for which there is no de novo synthesis in shrimp. The enrichment product acted by promoting somatic growth and increasing transfer of biochemical constituents needed by the ovary for develop

Using bone measurements to estimate the original sizes of bluefish (Pomatomus saltatrix) from digested remains

The ability to estimate the original size of an ingested prey item is an important step in understanding the community and population structure of piscivorous predators (Scharf et al., 1998). More specifically, knowledge of original prey size is essential for deriving important biological information, such as predator consumption rates, biomass of the prey consumed, and selectivity of a predator towards a specific size class of prey (Hansel et al., 1988; Scharf et al., 1997; Radke et al., 2000). To accurately assess the overall “top-down” pressure a predator may exert on prey community structure, prey size is crucial. However, such information is often difficult to collect in the field (Trippel and Beamish, 1987). Stomach-content analyses are the most common methods for examining the diets of piscivorous fish, but the prey items found are often thoroughly digested and sometimes unidentifiable. As a result, obtaining a direct measurement of prey items is frequently impossible.

Pacific Hake, Merluccius productus, Autecology: A Timely Review

Pacific hake, Merluccius productus, the most abundant groundfish in the California Current Large Marine Ecosystem (CCLME), is a species of both commercial significance, supporting a large international fishery, and ecological importance, connecting other species as both predator and prey. Coastal Pacific hake migrations are characterized by movements between northern summer feeding areas and southern winter spawning areas, with variations in annual abundance, distribution, and the extent of these movements associated with varying climate-ocean conditions. In general, warm (cool) years with enhanced (reduced) stratification and poleward (equatorward) transport are often related to good (poor) recruitment, increased (decreased) northward distribution, and reduced (enhanced) growth. However, the classic periodic pattern of annual migration and distribution may no longer be fully representative. Based on recent advances in the understanding of climate-ocean variability off the U.S. west coast, we hypothesize that the annual movements of Pacific hake are more responsive to climate-ocean variability than previously thought, and further, that changes observed in Pacific hake distributions may reflect long-term changes in climate-ocean conditions in the CCLME. Therefore, an updated model of these relations is key to effective monitoring and management of this stock, as well as to devising scenarios of future change in the CCLME as a result of climate variations. The current state of knowledge of the relationship between the Pacific hake and its environment is reviewed, highlighting emerging ideas compared to those of the past, and priorities for future research are suggested.

Beluga, Delphinapterus leucas, Habitat Associations in Cook Inlet, Alaska

A review of available information describing habitat associations for belugas, Delphinapterus leucas, in Cook Inlet was undertaken to complement population assessment surveys from 1993-2000. Available data for physical, biological, and anthropogenic factors in Cook Inlet are summarized followed by a provisional description of seasonal habitat associations. To summarize habitat preferences, the beluga summer distribution pattern was used to partition Cook Inlet into three regions. In general, belugas congregate in shallow, relatively warm, low-salinity water near major river outflows in upper Cook Inlet during summer (defined as their primary habitat), where prey availability is comparatively high and predator occurrence relatively low. In winter, belugas are seen in the central inlet, but sightings are fewer in number, and whales more dispersed compared to summer. Belugas are associated with a range of ice conditions in winter, from ice-free to 60% ice-covered water. Natural catastrophic events, such as fires, earthquakes, and volcanic eruptions, have had no reported effect on beluga habitat, although such events likely affect water quality and, potentially, prey availability. Similarly, although sewage effluent and discharges from industrial and military activities along Cook Inlet negatively affect water quality, analyses of organochlorines and heavy metal burdens indicate that Cook Inlet belugas are not assimilating contaminant loads greater than any other Alaska beluga stocks. Offshore oil and gas activities and vessel traffic are high in the central inlet compared with other Alaska waters, although belugas in Cook Inlet seem habituated to these anthropogenic factors. Anthropogenic factors that have the highest potential negative impacts on belugas include subsistence hunts (not discussed in this report), noise from transportation and offshore oil and gas extraction (ship transits and aircraft overflights), and water quality degradation (from urban runoff and sewage treatment facilities). Although significant impacts from anthropogenic factors other than hunting are not yet apparent, assessment of potential impacts from human activities, especially those that may effect prey availability, are needed.