No abstract

Glacial fjord habitats are undergoing rapid change as a result of contemporary global warming, yet little is known about how glaciers influence marine ecosystems. These ecosystems provide important feeding, breeding and rearing grounds for a wide variety of marine organisms, including seabirds of management concern. To characterize ocean conditions and marine food webs near tidewater glaciers, we conducted monthly surveys of oceanographic variables, plankton, fish and seabirds in Kenai Fjords, Alaska, from June to August of 2007 and 2008. We also measured tidal current velocities near glacial features. We found high sediment load from glacial river runoff played a major role in structuring the fjord marine ecosystem. Submerged moraines (sills) isolated cool, fresh, stratified and silt‐laden inner fjord habitats from oceanic influence. Near tidewater glaciers, surface layers of turbid glacial runoff limited availability of light to phytoplankton, but macrozooplankton were abundant in surface waters, perhaps due to the absence of a photic cue for diel migration. Fish and zooplankton community structure varied along an increasing temperature gradient throughout the summer. Acoustic measurements indicated that low density patches of fish and zooplankton were available in the surface waters near glacial river outflows. This is the foraging habitat occupied most by Kittlitz’s murrelet (Brachyramphus brevirostris), a rare seabird that appears to be specialized for life in glacially influenced environments. Kittlitz’s murrelets were associated with floating glacial ice, and they were more likely to occur near glaciers, in deeper water, and in areas with high acoustic backscatter. Kittlitz’s murrelet at‐sea distribution was limited to areas influenced by turbid glacial outflows, and where prey was concentrated near the surface in waters with low light penetration. Tidewater glaciers impart unique hydrographic characteristics that influence marine plankton and fish communities, and this has cascading effects on marine food webs in these ecosystems.

Fish larvae employ different feeding strategies depending on area and season of spawning and hatching of larvae. Feeding and growth of larvae of blue whiting Micromesistiuspoutussou and mackerel Sconiher scomhrus from Porcupine Bank and the Celtic Shelf Break, west of Ireland, were compared based on prey concentrations in the environment and larval feeding behaviour. Both species were adapted to different environmental conditions. The mesopelagic blue whiting spawned in oceanic water that was well mixed. It was characterized by low production and low prey densities with minimum prey densities <1.0 organism 1-'. Larvae of the Atlantic mackerel hatched later in the season in more productive water that was well stratified. Prey densities in the mackerel environment reached up to 100 1 ~ I . Blue whiting larvae displayed a rather random distribution in the water column. Mackerel larvae <7 mm standard length (L,) were concentrated above the thermocline, while larvae >7 mm traversed the thermocline into deeper layers. Mackerel larvae >5 mm L, displayed marked cannibalism, exceeding 70'X~ Daily ration calculated on the basis of gut contents was rather low in both species: between 2.6 and 50% in blue whiting, but only 0.6 to S.4Y0 in mackerel. The results are discussed in relation to the respective environment both species encounter during their early larval life. 'C 1997 The Fisheries Society of the British Irlea

Feeding of larval walleye pollock was examined with respect to density and distribution of micro-and mesozooplanktonic prey (<500 pm) at 6 depths in the oceanic domain of the Bering Sea to determine if first-feeding larvae select among available prey and to assess their foraging environment in comparison to other locations where walleye pollock spawn. At 30 m depth, walleye pollock larvae and copepod nauplii occurred at maximum densities of 106.6 ind. 100 m-3 and 26.3 ind. 1-', respectively. First-feeding walleye pollock larvae (3.5 to 6.5 mm) fed exclusively on copepod nauplii and copepod eggs. Among copepod nauplii, larvae selected for Metridia sp. and Microcalanus sp. but against Oithona similis, even though the latter was the most abundant prey taxon in the study area. Of the nauplii ingested by larval walleye pollock, there was evidence of selection for larger nauplii within Metridia sp., Microcalanus sp., and 0. sirnilis. In addition, larvae preferred Stages I11 to V of calanoid nauplii Larvae at 30 m depth had the highest incidence of feeding (97.5%) and number of prey items (4.6 larva-') ingested. Although the 30 m depth stratum prov~ded best physical and foraging conditions, the overall low percentage of feeding larvae and low numbers of prey consumed suggest that foraging conditions for larval walleye pollock at the time of sampling were below saturation feeding levels.

Short-term variability in vertical distribution and feeding of Atlantic mackerel (Scomber scombrus L.) larvae was investigated while tracking a larval patch over a 48-h period. The patch was repeatedly sampled and a total of 12,462 mackerel larvae were caught within the upper 100 m of the water column. Physical parameters were monitored at the same time. Larval length distribution showed a mode in the 3.0 mm standard length (SL) class (mean abundance of 3.0 mm larvae x =75.34 per 100 m 3 , s=34.37). Highest densities occurred at 20-40 m depth. Larvae <5.0 mm SL were highly aggregated above the thermocline, while larvae ≥5.0 mm SL were more dispersed and tended to migrate below the thermocline. Gut contents of 1,177 mackerel larvae (2.9-9.7 mm SL) were analyzed. Feeding incidence, mean number (numerical intensity) and mean dry weight (weight-based intensity) of prey items per larval gut were significantly dependent on larval size. However, while weight-based feeding intensities continued to increase with larval length, numerical intensity peaked at 4-4.9 mm SL, indicating a shift in the larval diet. While first-feeding larvae relied most heavily on copepod nauplii and eggs, larvae ≥5.0 mm SL initiated piscivorous feeding. All identifiable fish larvae were Atlantic mackerel. Thus, the piscivory was cannibalism. Larval feeding incidence and numerical feeding intensities peaked during daytime and were reduced at night. Daily ration estimates for first-feeding mackerel larvae <4.0 mm SL were extremely low x =1.43% body dry weight, but increased dramatically at 5.0 mm SL, i.e., at the onset of cannibalism, reaching >50% body dry weight in larva ≥8.0 mm SL.

No abstract

This study examines potential interactions among the environmental variables likely to affect larval walleye pollock Theragra chalcogramma feeding in the sea. Walleye pollock larvae were sampled from Shelikof Strait, Gulf of Alaska, and from the eastern Bering Sea, with corresponding environmental data. Variables used in our study were time spent feeding, seawater temperature, light, prey density, wind speed and standard length of the larvae. We applied an additivity test to detect the presence of potential interactions among these variables and adopted an expansion of generalized additive models (GAMs) that allowed the inclusion of interaction terms in a non-parametric regression analysis. After testing all possible 2-way interactions among these variables, we found 4 significant terms: (1) time feeding-standard length, (2) temperature-light, (3) light -wind speed and (4) prey density-standard length. The most influential interaction term was the light -wind speed interaction, which caused a decrease of the model-generalized cross-validation (GCV, whereby lower values indicate more parsimonious models) from 0.0402 (for a model with all variables but no interaction term) to 0.0290 (for a model with all variables and this interaction), and an increase of the model explained variance by 7% (R 2 = 0.89 versus 0.82). This result indicates that the effect of wind speed (turbulence) on larval walleye pollock feeding is dependent upon the amount of light available. This may be due to vertical movements in the water column by both walleye pollock larvae and their prey in response to turbulence. KEY WORDS: Larval fish feeding · Walleye pollock larvae · Generalized additive models Resale or republication not permitted without written consent of the publisher

In March/April 1994 a hydrographic and ichthyoplankton survey was conducted in the area of Porcupine Bank, west of Ireland, to study the distribution and feeding of blue whiting larvae. The Porcupine Bank area was characterized by 2 distinguishable water masses separated by a thermal front: (1) the warm and saline waters of the secondary shelf edge current (sSEC) and (2) the cooler and less saline waters above Porcupine Bank associated with an anticyclonic circulation. Highest concentrations of recently hatched larvae <4.0 mm standard length (SL) occurred in waters of the sSEC, while larvae >4.0 mm SL were more abundant in the cool waters above Porcupine Bank and larvae >5.0 mm SL were exclusively found above the bank. Copepod nauplii density was extremely low, with an overall mean density of 5.2 1-' Nauplii were most abundant in the water of the sSEC. However, proportionally more large calanoid nauplii were observed above Porcupine Bank. The diet of blue whiting larvae also varied among water masses, with larvae having higher feeding incidences and numerical feeding intensities in the sSEC but significantly higher feeding intensities by weight and, thus, a higher energetic gain in the waters above the bank. These contrasts resulted from differences in the composition of the larval diet. Larvae in the sSEC preyed heavily on tintinnids and small cyclopoid nauplii, while larvae above Porcupine Bank also ingested large calanoid nauplii. In addition, blue whiting larvae had different selectivity patterns depending on the foraging environment, with larvae retained above Porcupine Bank selecting strongly for calanoid nauplii, which were never selected for by larvae in the sSEC. Instead those larvae demonstrated a preference for smaller cyclopoid nauplii. In summary, we conclude that in 1994 blue whiting larvae benefited from being retained above Porcupine Bank not only by maintaining a close horizontal distribution but also by utilizing the more favorable feeding environment above the bank.