Kenneth O. Coyle

Foraging in a fractal environment: Spatial patterns in a marine predator-prey system

Spatial relationships between predators and prey have important implications for landscape processes and patterns. Highly mobile oceanic birds and their patchily distributed prey constitute an accessible model system for studying these relationships. High-frequency echosounders can be used together with simultaneous direct visual observations to quantitatively describe the distributions of seabird consumers and their resources over a wide range of spatial scales, yielding information which is rarely available in terrestrial systems.Recent fine-scale investigations which have used acoustics to study the distribution of foraging marine birds have reported weak or ephemeral spatial associations between the birds and their prey. These results are inconsistent with predictions of optimal foraging, but several considerations suggest that traditional foraging models do not adequately describe resource acquisition in marine environments. Relative to their terrestrial counterparts, oceanic ‘landscapes’ are structurally very simple, but they generally lack visual cues about resource availability.An emerging view assumes that perceptually constrained organisms searching for food in multiscale environments should respond to patterns of resource abundance over a continuum of scales. We explore fractal geometry as a possible tool for quantifying this view and for describing spatial dispersion patterns that result from foraging behavior. Data on an Alaskan seabird (least auklet [Aethia pusilla]) and its zooplanktonic food resources suggest that fractal approaches can yield new ecological insights into complex spatial patterns deriving from animal movements.

Climate-related differences in zooplankton density and growth on the inner shelf of the southeastern Bering Sea

Abstract Zooplankton abundance and biomass were measured during spring and late summer on the inner shelf of the southeastern Bering Sea in years of climate extremes. Samples were taken during late spring and late summer of three years: 1997, a year of moderate spring ice cover and unusually warm, calm summer conditions; 1998, a year of warm, but stormy summer conditions with very little spring ice cover; and 1999, a year of extensive spring ice cover, cold spring conditions and storms during spring and summer. Mean water column temperature was significantly lower during June 1999 than June 1998 and 1997. Copepod abundance and biomass during June were correlated with mean water column temperature and mean temperature below the thermocline. Mean calanoid abundance during June 1999 was 8–52% of the mean abundance during 1998 and 1997. Significantly lower abundances during June 1999 were observed for Calanus marshallae , Acartia spp., Pseudocalanus spp. and calanoid nauplii. Significant interannual differences in mean water column temperature and calanoid abundance during late summer (late July–early September) were not detected. The Hirst–Lampitt equations were used to estimate the mean daily copepod production during the warm and cold years. The mean production estimate during warm conditions was 13 mg C m −2 d −1 , with a range of 3–37 mg C m −2 d −1 , similar to previous estimates. Production estimates during the cold spring, 1999, were 3–4% of the production during warm periods. Assuming a 35% gross growth efficiency, calanoids could consume an average of 37 mg C m −2 d −1 during warm periods, but only about 2.6 mg C m −2 d −1 during a cold spring. Comparison of the above estimates to concurrent measures of primary production indicate that during warm, calm seasons, calanoids could remove most or all of the available water column primary production, thus indicating that calanoids may have been food limited. During cool conditions, the calanoids could remove only about 3% of the estimated daily primary production. Lower post-bloom primary production rates and higher calanoid grazing rates may result in substantially lower annual carbon flux to benthic communities on the inner shelf during warm calm years relative to cold years with extensive spring ice cover.

Interannual and decadal variability in zooplankton communities of the southeast Bering Sea shelf

The southeastern Bering Sea shelf ecosystem is an important fishing ground for fin- and shellfish, and is the summer foraging grounds for many planktivorous seabirds and marine mammals. In 1997 and 1998, Northern Hemisphere climate anomalies affected the physical and biological environment of the southeastern Bering Sea shelf. The resulting anomalous conditions provided a valuable opportunity to examine how longer-term climate change might affect this productive ecosystem. We compared historical and recent zooplankton biomass and species composition data for the southeastern Bering Sea shelf to examine whether or not there was a response to the atmosphere–ocean–ice anomalies of 1997 and 1998. Summer zooplankton biomass (1954–1994) over the southeastern shelf did not exhibit a decline as previously reported for oceanic stations. In addition, zooplankton biomass in 1997 and 1998 was not appreciably different from other years in the time series. Spring concentrations of numerically abundant copepods (Acartia spp., Calanus marshallae, and Pseudocalanus spp.), however, were significantly higher during 1994–1998 than 1980–1981; spring concentrations of Metridia pacifica and Neocalanus spp. were not consistently different between the two time periods. Neocalanus spp. was the only taxon to have consistent differences in stage composition between the two time periods—CV copepodites were much more prevalent in May of the 1990s than early 1980s. Since relatively high zooplankton concentrations were observed prior to 1997, we do not attribute the high concentrations observed in the summers of 1997 and 1998 directly to the acute climate anomalies. With the present data it is not possible to distinguish between increased production (control from below) and decreased predation (control from above) to explain the recent increase in concentrations of the species examined.

The abundance and distribution of euphausiids and zero-age pollock on the inner shelf of the southeast Bering Sea near the Inner Front in 1997–1999

Abstract Acoustic data and net samples were collected during late spring and early fall 1997–1999 to assess zooplankton and micronekton abundance and distribution relative to the Inner Front at three sampling grids (Port Moller, Cape Newenham and Nunivak Island) on the inner shelf of the southeast Bering Sea. Epibenthic scattering layers were observed during May–June and August–September in all three years. Acoustic data were scaled to euphausiid biomass using target strength models. Mean euphausiid biomass determined acoustically for each transect line was 0.7– 21 g m −2 , with most values below 5 g m −2 . There was no consistent relationship between the distribution and biomass of euphausiids and the location of the Inner Front. Zero age pollock were observed on the inner shelf in August–September during all years, but were confined primarily to the stratified side of the Inner Front and to the frontal regime. The acoustic data for pollock were scaled to biomass using laboratory measurements of gas bladder dimensions and target strength models. Acoustic determinations of mean transect biomass for euphausiids did not differ from literature values for the inner shelf of the southeast Bering Sea, and pollock biomass on the inner shelf did not differ from that around the Pribilof Islands. Despite recent anomalies in climate and oceanographic conditions on the inner shelf, and high mortality of shorttail shearwaters during 1997, we found no evidence of significant interannual differences in the biomass of euphausiids or zero-age pollock on the inner shelf of the southeast Bering Sea.

Water column sound scattering and hydrography around the Pribilof Islands, Bering Sea

Abstract Water column structure around the Pribilof Islands is a complex mixture of salinity and temperature fronts. Elevated fluorescence and sound-scattering patterns were often observed in the upper 30 m around frontal regions during both 1987 and 1988. Salinity fronts with associated elevated sound scattering were particularly intense near the surface during 1987. During both years sound scattering was most intense over the shelf, decreased over the slope and was lowest over the deep basin. An epibenthic sound-scattering layer was often present but its distribution did not appear to be related to water column structure. Water column scattering patterns in 38- and 200-kHz data were similar, but epibenthic scattering peaks present in the 200-kHz data were absent from the 38-kHz data. Net samples indicate that a portion of the water column scattering was probably caused by large gelatinous zooplankton, while a portion of the 200-kHz peaks in the epibenthic layers probably resulted from sound reflected by euphausiid swarms.

Variations in Egg Production Rates by Pseudocalanus Spp. In a Subarctic Alaskan Bay During the Onset of Feeding by Larval Fish

Weekly estimates of female abundance and daily egg production rates were made for Pseudocalanus spp. during 1987 and 1988 in Auke Bay, southeastern Alaska. The sampling, which included measurements in primary production, was done during April and May when larval fish initiate feeding on copepod nauplii. The objective of the study was to examine the relative importance of female abundance and daily egg production rates for Pseudocalanus spp. in determining production of nauplii during the study period. In both years primary production rates exceeded 500 mg C m-2 d-1. At this level of primary production Pseudocalanus spp. should not have been food-limited in either year. The average egg production rates per individual female during the 2 years were identical: 2.6 eggs female1 d-~. The average populations of female Pseudocalanus were 4,765 and 7,656 individuals m-2 in 1987 and 1988, respectively. The larger populations of female Pseudocalanus spp. in 1988 resulted in higher overall egg production and better feeding conditions for larval fish. Thus, during the period when larval pollock began feeding on nauplii, female copepod abundance was more important than temporal differences in egg production rates in determining production of nauplii. A complete understanding of the copepod-larval fish relationship in Auke Bay will require explanation of the causes for interannual variability in abundance of adult female Pseudocalanus spp. In high latitude ecosystems several species of copepods and fishes produce their young during the spring phytoplankton bloom. At the onset of feeding, larvae of many fishes including pollock, Theragra chalcogramma (Pallas), feed on copepod nauplii. Fish larvae have little stored energy reserves and must encounter appropriate concentrations of suitably sized nauplii in order to feed and survive (Checkley, 1982; Cohen and Lough, 1983; Haldorson, Paul, Sterritt, and Watts, 1989; Paul, 1983). During 1987 and 1988 daily growth rates were measured for larval pollock in Auke Bay, southeastern Alaska (Haldorson, Paul, Sterritt, and Watts, 1989) as part of the APPRISE (Association of Primary Production and Recruitment in Subarctic Ecosystems) project. While pollock larvae were present throughout much of the spring period, most of the years pollock egg production occurred during the spring diatom bloom (Haldorson, Watts, Sterritt, and Pritchett, 1989). The main prey of first-feeding pollock larvae was copepod nauplii (Sterritt, 1989). Typically, nauplii were scarce in early spring and increased in number during the spring phytoplankton bloom (Haldorson, Paul, Sterritt, and Watts, 1989). Major cohorts of pollock larvae grew at 0.16 in 1987 versus 0.22 mm d-l in 1988. In 1988, nauplii were more abundant than in 1987 and Haldorson, Paul, Sterritt, and Watts (1989) proposed that the 1988 cohort of pollock larvae grew faster because there was more food. Theoretically, survival of fish larvae could be enhanced if the larvae initiated feeding when production of copepod nauplii was maximal (Cushings (1975) match-mismatch hypothesis). The hypothesis explored in this report is that interannual difference in copepod egg production rates was an important factor in determining abundance of nauplii. In Auke Bay, the most abundant copepod type is Pseudocalanus spp. (Coyle et al., 1990) and its young are a major prey species of pollock (Sterritt, 1989). Previously, there have been several studies of survival and growth of larval pollock relative to prey abundance (Haldorson, Paul, Sterritt, and Watts, 1989; Haldorson, Watts, Sterritt, and Pritchett, 1989; Incze et al., 1990; Nishiyama and Hirano, 1985; Paul, 1983), but there has been little effort allocated to examination of the factors which

Timing of Spawning of Thysanoessa Raschii (Euphausiacea) and Occurrence of Their Feeding-Stage Larvae in an Alaskan Bay

ABSTRACT The timing of spawning and onset of feeding for euphausiid larvae in Auke Bay, Alaska, was monitored for 3 years. Thysanoessa raschii was the most abundant euphausiid species in the study area. In all years the eggs were most abundant in the plankton soon after the major spring phytoplankton bloom was completed. During all 3 years of observation, the major cohort of stage I calyptopis, which is the first feeding stage, appeared in the plankton after the primary diatom bloom was finished. When the calyptopes initiated feeding, the water column was physically stratified and low nutrient availability limited primary production in the photic zone. The results suggest that euphausiid larvae in Auke Bay are dependent on secondary phytoplankton blooms for food, rather than the major spring diatom bloom. The concentrations of nauplii were about 70,000·m―2 in 1986, 40,000·m―2 in 1987, and 140,000·m―2 in 1988. Concentrations of calyptopes peaked at about 40,000·m―2 in 1986, 10,000·m―2 in 1987, and 70,000·m―2 in 1988. Maximum concentrations of furciliae were 730 to 1,660·m―2 in 1988, 1,037·m―2 in 1986, and 10 to 19·m―2 in 1987.

Population Fluctuations of Jellyfish in the Bering Sea and Their Ecological Role in This Productive Shelf Ecosystem

A long-term fisheries monitoring program operating in the southeastern Bering Sea detected a biomass increase of large jellyfish in the 1990s. However, medusa biomass declined to lower levels after 2000, but then increased once again in 2009. Similar population fluctuations are revealed in other monitoring efforts that extend to the northeast Bering Sea and to the west in Russian waters. Decadal oscillations in climate, rather than overfishing or other anthropogenic factors, are thought to be responsible for these trends. This case study of Bering Sea jellyfish blooms demonstrates that apparent increases in jellyfish populations may not necessarily be sustained and that increases may occur in response to climate variability. Herein we review what is known about the abundance and distribution of the dominant species of jellyfish in the Bering Sea and their potential interactions with other parts of the ecosystem, particularly those of interest to humans.

Arctic ampeliscid amphipods: three new species

ABSTRACT Three new amphipod species in the family Ampeliscidae, Ampelisca erythrorhabdota, Byblis robustus, and Byblis frigidis, are described from the shallow shelf waters of the northern Bering and southern Chukchi seas. Ampelisca erythrorhabdota is most closely related to Ampelisca macrocephala and differs mainly in the color patterns of unpreserved specimens and size at maturity. Byblis frigidis differs from its closest relative, Byblis breviramus, in having longer antennae, spines on both edges of article 6, pereiopod 7, and in lacking the enlarged lateral spine on the end of the peduncle of uropod 1. Byblis robustus differs from other Byblis in having antennae of almost equal length and a distinctly triangular shape of the posterior lobe of pereiopod 7.

Abundance and Taxa Composition of Copepod Nauplii Over Southeastern Bering Sea Deep Water Spawning Grounds of Walleye Pollock, Theragra Chalcogramma

This project examined the abundance, taxa, stage and size of copepod nauplii available as prey for walleye pollock larvae in its deep water spawning grounds of the southeastern Bering Sea. Nauplii of the genera Oithona, Eucalanus, and Metridia were the most commonly encounted types while Pseudocalanus and Microcalanus were rare. There was marked interannual variation in the taxa and abundance of copepod nauplii coexisting with the pollock larvae. Stages I-IV of Oithona were present at 0 to 8 l-1 at different depths. Individual stages III-VI of Metridia nauplii occurred at 0 to 6 l-1, while Eucalanus nauplii were present at 0.1 to 9 l-1, depending on stage and depth. Nauplii III-IV of the genera Pseudocalanus and III-VI Microcalanus were usually present at less than 1 l-1. In 1992, 54 to 56% of the nauplii had body lengths of 150-350 μm which are typical of those found in stomachs of first-feeding pollock larvae. In 1993 corresponding estimates were 64 to 68%. Copepod nauplii with body lengths of 150-350 μm were generally present at 1 to 15 l-1, but at one station their counts exceeded 20 l-1.