© 2005 International Council for the Exploration of the Sea
Analyses of Bering Sea bottom-trawl surveys in Norton Sound: absence of regime shift effect on epifauna and demersal fish
a Alaska Department of Fish & Game, Commercial Fishery Division Anchorage, AK 99518, USA
b Alaska Department of Fish & Game, Commercial Fishery Division Kodiak, AK 99615, USA
c Alaska Department of Fish & Game, Commercial Fishery Division Nome, AK 99762, USA
*Correspondence to T. Hamazaki: tel: +1 907 267 2158; fax: +1 907 267 2442. e-mail: hamachan_hamazaki{at}fishgame.state.ak.us.
This study retrospectively examined evidence of ocean climate regime shift effects on epifauna and demersal fish of Norton Sound, Alaska, northeast Bering Sea, based on triennial bottom-trawl surveys from 1976 to 2002. Throughout the period, benthic fauna was dominated by sea stars (48–78%), followed by cods (5–19%), flatfish (5–15%), sculpins (1.5–7%), and crabs (2–6%). From 1976 to 2002, the cpue index of total species increased exponentially (4.5% y–1) by threefold with some declines in 1991 and 1999. The increase was also observed in sea stars (5.1% y–1), flatfish (6.1% y–1), and crabs (2.5% y–1). However, trends of cods and sculpins were mixed. Regression analysis showed the cpue index of total species to be positively correlated with survey years and bottom-water temperature. However, bottom-water temperature, when considered by itself, was not significant. Results suggest that regime shifts caused biomass increases of Norton Sound epifauna and demersal fish.
Keywords: Bering Sea, demersal fishes, epifauna, Norton Sound, regime shift
Received 29 November 2004; accepted 14 June 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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It is widely recognized that the Bering Sea has been experiencing abrupt ocean-wide climatic and oceanographic shifts (regime shifts), occurring on a decadal scale in 1925, 1947, 1977, 1989, and possibly in 1998 (Graham, 1994; Miller et al., 1994; Hare and Mantua, 2000; Benson and Trites, 2002). These shifts are not limited to climatic and oceanographic conditions. As biota of the Bering Sea is largely influenced by physical and oceanographic conditions, many biological indices (e.g. fishery catch statistics) also showed abrupt changes coincident with regime shifts. From those associations, the 1977–1988 period is generally considered a warm regime with high biological productivity, whereas the 1989–present period is a cold regime with low biological productivity (Hare and Mantua, 2000; Benson and Trites, 2002).
Mechanisms describing the effect of regime shifts on biota are understood to involve ocean currents, and cascading (Niebauer et al., 1981, 1995; Gargett, 1997; Sugimoto and Tadokoro, 1997; Francis et al., 1998; McGowan et al., 1998; Anderson and Piatt, 1999; Hunt et al., 2002). Changing climatic conditions alter oceanographic current patterns and the spatial/temporal distribution of sea ice, which alter spatial/temporal distribution of water temperature and nutrient upwelling, which affect spatial/temporal distribution of phytoplankton and water-temperature sensitive species, biological productivity, species composition, and ecosystem structure. These understandings, however, are based largely on studies in the southeast Bering Sea and the Gulf of Alaska where ocean currents have significant impacts on biota.
The Bering Sea consists of several regions with distinct water masses (Takenouti and Ohtani, 1974) each with their own unique ecological characteristics of physical oceanographic environment, biological community composition, and ecosystem function (NRC, 1996). Since not all regions are significantly influenced by ocean currents, mechanisms of the regime shifts may differ among regions, or the regime shift effects from the same mechanism could differ among regions. Specifically, if the effects of regime shifts are only ocean-current-driven, then regime shifts have little effect on biota of Bering Sea regions where ocean currents are not prevalent. Thus far, the effects of regime shifts have not been examined in the other Bering Sea regions, primarily because long time-series data are absent (NRC, 1996).
Norton Sound of the north Bering Sea is one of a few places where long time-series data are available. Ecological characteristics of Norton Sound differ from those of the southeast Bering Sea. Whereas the southeast region is characterized as an oceanic ecosystem, surrounded by strong oceanic currents, such as Commander Currents, Bering Slope Currents, and the Alaskan Stream, Norton Sound is characterized as an inshore ecosystem, consisting of the inshore Norton Sound Water Mass, a diversion of the Alaska Coastal Water Current (Takenouti and Ohtani, 1974; Nelson et al., 1981). In the southeast, the currents influence location and duration of sea ice that serves as a major source of freshwater, nutrients, and water mixing-major determinants of biological productivity. In contrast, Norton Sound receives most of its freshwater and nutrients from the Yukon and Kuskokwim Rivers and other small rivers across the Seward Peninsula; most water mixing is driven by tides and winds (Goering and Iverson, 1981; Coachman, 1986).
The benthic fauna of Norton Sound also differs from that of the southeast Bering Sea. Whereas the southeast is dominated by demersal fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), yellowfin sole (Limanda aspera), and rock sole (Lepidopsetta spp.) (Conners et al., 2002), Norton Sound is dominated by invertebrates, especially the sea star, Asterias amurensis, a generalist predator, which mainly inhabits the depth range of 0–40 m along the coast across the Bering and Chukchi Seas (Sloan, 1980; Jewett and Feder, 1981; Fukuyama and Oliver, 1985). The southeast Bering Sea has the highest primary and secondary productivity, whereas Norton Sound has the lowest (NRC, 1996).
The above differences suggest that the effects of regime shifts on benthic fauna in Norton Sound would compare with the southeast Bering Sea. The objective of this study is to retrospectively examine the presence and extent of regime shift effects on Norton Sound epifauna and demersal fish. As Norton Sound is an inshore system, we hypothesize that any effect of a regime shift on the Norton Sound benthic epifauna and demersal fish would be minimal.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Norton Sound epifauna and demersal fish have been monitored triennially since 1976 in response to the development of a red king crab (Paralithodes camtschaticus) pot fishery initiated in 1977 (Brennan, 2003). The triennial trawl survey has been conducted by the National Marine Fisheries Service (NMFS) (1976–1991) and by the Alaska Department of Fish & Game (ADF&G) (1996 to the present) to monitor the distribution and abundance of red king crab and demersal fish (e.g. flounders). NMFS and ADF&G conducted trawl surveys with a similar format, except for trawl gear used, sampling schedule, and total area trawled (Table 1, Figure 1; Fair, 1998). The NMFS survey was conducted over the entire Norton Sound, whereas the ADF&G survey was limited to areas where the commercial crab pot fishery operates (Figure 1). The trawl gear used by NMFS (83–112 Eastern) is more efficient at catching walleye pollock (Theragra chalcogramma) and Pacific cod (Gadus macrocephalus), while gear used by ADF&G (400 Eastern) is more efficient at catching arrowtooth flounder (Atheresthes stomias) and flathead sole (Hippoglossoides elassodon) (Szalay and Brown, 2001). However, this difference is considered negligible in examining total abundance estimates (Conners et al., 2002).
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In Norton Sound sampling stations were evenly spaced to represent an approximately 18.5 x 18.5 km (10 x 10 nautical miles) square grid (Figure 1). At each station, a trawl was towed once for approximately 2.6 km (1.5 miles; NMFS), or 1.9 km (1.0 miles; ADF&G). When the first tow was unsuccessful, a second tow was completed. For each tow, all red and blue king crabs (Paralithodes camtschaticus, P. platypus, respectively), Pacific halibut (Hippoglossus stenolepis), and Pacific cod (Gadus macrocephalus) were separated, counted, and weighed. For the remainder of each catch, two or three subsample buckets were taken, sorted to the lowest possible taxon, weighed, and counted. Total catch weight of each taxon was estimated by multiplying its subsample percentage weight with total catch weight (excluding red and blue king crabs, Pacific halibut, and Pacific cod). Bottom-water temperature was also collected at each site. Catch weights of each haul were converted into catch per unit effort (cpue): weight per area swept (kg km–2) calculated as effective width of the net times distance towed. The cpue index of each taxon was calculated as the geometric mean of non-zero cpue multiplied by the percentage of non-zero hauls for each taxon (Pennington, 1983; Conners et al., 2002). In this retrospective analysis, only stations within ADF&G surveyed areas were used for calculation of the cpue index.
To examine influences of regime shifts, a multiple log-linear regression model was constructed, in which the log-transformed cpue index was regressed with survey year and bottom temperature: log(cpue) = ß0 + ß1(year – 1976) + ß2(bottom temperature). This model has a biologically reasonable, exponential growth trajectory and multiplicative lognormal error structure.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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From 1976 to 2002, 322 taxonomic groups were captured and identified (224 species identified); these were combined into 48 family/genus groups and further into 9 groups. Five major groups represented the benthic fauna: sea stars (49–78%), cods (4–31%), flatfish (4–10%), sculpins (2–8%), and crabs (2–8%) (Tables 2, 3, Figure 2). The percentage of sea stars tended to be lower during the warm regime years (1976–1988) than during cold regime years (1989–2002) (mean: 58.0 vs. 67.8%).
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The cpue index of total species generally trended upwards, trending down in 1991 and 1999 (Table 3). The trend differed for each group. An increasing trend was observed for sea stars, crabs, and flatfish, but trends of cods and sculpins were mixed. Regression analyses showed that the cpue index for total species, sea stars, crabs, and flatfish was significantly positively correlated (p < 0.05) with survey years, with mean annual increase rates 4.5%, 5.1%, 2.6%, and 6.1%, respectively (Table 4). Annual mean bottom temperature showed a mixed trend: a significant positive correlation with total species and a negative correlation with crabs (p < 0.05) (Tables 3, 4). However, models considering temperature alone did not show significant correlations (p > 0.05).
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At family/genus level, 30 out of 48 groupings showed increasing trends over the years (Spearman correlation coefficient: rs > 0.1); however, only five groups had statistically significant trends (p < 0.05). This lack of statistical significance was due to sparse and highly variable data.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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In order to appropriately assess the effects of regime shifts on biota, data should be collected in a continual and consistent manner, should reflect characteristics of the biota, and confounding effects of the fishery should be minimized. From these perspectives, we acknowledge that Norton Sound triennial survey data are not ideal. The survey interval is too sparse (9 survey years in 26 years) to examine detailed trends. The effectiveness of trawl gear for assessing the true abundance of invertebrates is also unclear. Catch of invertebrates may be influenced by survey techniques (M. Elizabeth Conners, pers. comm.). More significantly, this study assumes that the 1976 survey reflects benthic faunal characteristics of the period before the 1977 regime shift. This assumption is unverifiable though reasonable. Norton Sound did not have a major groundfish fishery until 1977, so the 1976 benthic fauna is considered unexploited. Furthermore, since the crab pot fishery is small with minimum bycatch (Brennan et al., 2003), the fluctuations are largely the result of natural causes.
Despite the above deficiencies, a clear trend emerged. The Norton Sound benthic biomass increased since 1976 and the benthic fauna remained dominated by sea stars (Figure 2, Table 3). Interestingly, this trend was comparable to the southeast Bering Sea: north of Unimak Island, Bristol Bay, and the Pribilof Islands (Conners et al., 2002). While species composition and abundance differed between areas, in all the three areas, cpue of total species, demersal fish, and non-crab benthic invertebrates was stable from 1960 to the early 1970s, increased threefold to fivefold from the late 1970s to the early 1990s, then declined somewhat in the late 1990s. Furthermore, in the southeast Chukchi Sea and Kotzebue Sound located just north of Norton Sound, Feder et al. (2005) compared 1976 and 1998 trawling survey results. They found that the biomass of the most dominant species/groups increased by twofold or more in 1998, specifically sea stars (Asterias amurensis), snow crabs (Chionoecetes opilio), tunicates, and sea urchins (Strongylocentrotus droebachiensis), while little change was observed in epifaunal invertebrate compositions. These surveys indicate that benthic fauna biomass increased significantly without major species changes across various Bering Sea regions since the 1977 shift.
However, it is unclear if those consistent responses were caused by the same mechanisms. Ecosystem characteristics differ between the inshore Norton Sound ecosystem and the ocean-current-dominated southeast Bering Sea ecosystem, so ocean-current-driven regime shift effect mechanisms (e.g. Francis et al., 1998; McGowan et al., 1998; Hunt et al., 2002) would not be applicable in Norton Sound. Further research to explore factors affecting productivity of the Norton Sound benthic fauna is needed to enhance our understanding of the structure and function of this ecosystem and its relationship to regime shifts.
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Acknowledgements |
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This work was the result of long-term survey efforts by NMFS and ADF&G. We thank all the field crews who participated in the surveys. We also thank Elizabeth Conners and Bernard A. Megrey, NMFS Alaska Fisheries Science Center, and Stephen Jewett and Howard Feder, University of Alaska Fairbanks, for their critical review of the manuscript.
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References |
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