Northeast US Shelf Research Articles

Event scale and persistent drivers of fish and macroinvertebrate distributions on the Northeast US Shelf

Event scale and persistent drivers of fish and macroinvertebrate distributions on the Northeast US Shelf

Disentangling the effects of climate, abundance, and size on the distribution of marine fish: an example based on four stocks from the Northeast US shelf

AbstractClimate change and fishing can have major impacts on the distribution of natural marine resources. Climate change alters the distribution of suitable habitat, forcing organisms to shift their range or attempt to survive under suboptimal conditions. Fishing reduces the abundance of marine populations and truncates their age structure leading to range contractions or shifts. Along the east coast of the United States, there have been major changes in fish populations due to the impacts of fishing and subsequent regulations, as well as changes in the climate. Black sea bass, scup, summer flounder, and winter flounder are important commercial and recreational species, which utilize inshore and offshore waters on the northeast shelf. We examined the distributions of the four species with the Northeast Fisheries Science Center trawl surveys to determine if the along-shelf centres of biomass had changed over time and if the changes were attributed to changes in temperature or fishing pressure through changes in abundance and length structure. Black sea bass, scup, and summer flounder exhibited significant poleward shifts in distributions in at least one season while the Southern New England/Mid-Atlantic Bight stock of winter flounder did not shift. Generalized additive modelling indicated that the changes in the centres of biomass for black sea bass and scup in spring were related to climate, while the change in the distribution of summer flounder was largely attributed to a decrease in fishing pressure and an expansion of the length–age structure. While the changes in ocean temperatures will have major impacts on the distribution of marine taxa, the effects of fishing can be of equivalent magnitude and on a more immediate time scale. It is important for management to take all factors into consideration when developing regulations for natural marine resources.

Predation on Prerecruits Can Delay Rebuilding of Depleted Cod Stocks

Predation by clupeoid species (e.g., Atlantic herring, Clupea harengus Linnaeus, 1758) on the early life stages of gadoids is hypothesized to be an important source of mortality, especially for Atlantic cod, Gadus morhua Linnaeus, 1758. Adult cod also prey on herring, creating a positive feedback process (trophic triangle) that may result in alternative dominance patterns of cod or herring depending on the relative levels of mortality. We estimated the effect of herring on cod recruitment with a predator-dependent Ricker stock-recruitment function, fit to time-series data. In this formulation, herring gradually reduce the per capita recruitment rate of cod, but this predation does not result in depensatory dynamics. We incorporated this predator-dependent recruitment function into a multispecies length-based model (LeMans) of Georges Bank, northeast US shelf, to investigate the consequences of predation mortality for the recovery of cod from overfishing. In LeMans, all fished species undergo the same size-dependent fishing mortality, following a logistic selection curve. We investigated two fishing patterns: a selective (L50 = 65 cm) and an unselective fishery (L50 = 15 cm). The effect of predation on prerecruits was more pronounced under selective fishing because herring were spared as a result of their small size. In rebuilding scenarios with selective fishing, cod started at low abundance and herring at high abundance. Without fishing, cod could rebuild in 10 yrs even with predation mortality before recruitment. In contrast, with low levels of fishing mortality, rebuilding took 25 yrs and even longer with such predation. These results suggest that predation on prerecruits and fishing can combine to delay rebuilding of depleted cod stocks.

Silver hake tracks changes in Northwest Atlantic circulation

Recent studies documenting shifts in spatial distribution of many organisms in response to a warming climate highlight the need to understand the mechanisms underlying species distribution at large spatial scales. Here we present one noteworthy example of remote oceanographic processes governing the spatial distribution of adult silver hake, Merluccius bilinearis, a commercially important fish in the Northeast US shelf region. Changes in spatial distribution of silver hake over the last 40 years are highly correlated with the position of the Gulf Stream. These changes in distribution are in direct response to local changes in bottom temperature on the continental shelf that are responding to the same large scale circulation change affecting the Gulf Stream path, namely changes in the Atlantic meridional overturning circulation (AMOC). If the AMOC weakens, as is suggested by global climate models, silver hake distribution will remain in a poleward position, the extent to which could be forecast at both decadal and multidecadal scales.

Network structure and robustness of marine food webs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 273:291-302 (2004) - doi:10.3354/meps273291 Network structure and robustness of marine food webs Jennifer A. Dunne1,4,*, Richard J. Williams2,4, Neo D. Martinez3,4 1Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501, USA 2National Center for Ecological Analysis and Synthesis, University of California Santa Barbara, 735 State St., Suite 300, Santa Barbara, California 93101-3351, USA 3Center for Applied Mathematics, 657 Frank HT Rhodes Hall, Cornell University, Ithaca, New York 14853, USA 4Pacific Ecoinformatics and Computational Ecology Lab, Rocky Mountain Biological Laboratory, PO Box 519, Crested Butte, Colorado 81224, USA *Email: jdunne@santafe.edu ABSTRACT: Previous studies suggest that food-web theory has yet to account for major differences in food-web properties of marine versus other types of ecosystems. We examined this issue by analyzing the network structure of food webs for the Northeast US Shelf, a Caribbean reef, and Benguela, off South Africa. The values of connectance (links per species2), link density (links per species), mean chain length, and fractions of intermediate, omnivorous, and cannibalistic taxa of these marine webs are somewhat high but still within the ranges observed in other webs. We further compared the marine webs by using the empirically corroborated niche model that accounts for observed variation in diversity (taxon number) and complexity (connectance). Our results substantiate previously reported results for estuarine, fresh-water, and terrestrial datasets, which suggests that food webs from different types of ecosystems with variable diversity and complexity share fundamental structural and ordering characteristics. Analyses of potential secondary extinctions resulting from species loss show that the structural robustness of marine food webs is also consistent with trends from other food webs. As expected, given their relatively high connectance, marine food webs appear fairly robust to loss of most-connected taxa as well as random taxa. Still, the short average path length between marine taxa (1.6 links) suggests that effects from perturbations, such as overfishing, can be transmitted more widely throughout marine ecosystems than previously appreciated. KEY WORDS: Food webs · Network structure · Marine ecosystems · Robustness · Connectance · Niche model · Biodiversity loss Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 273. Online publication date: June 08, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Trophic ecology of Atlantic cod Gadus morhua on the northeast US continental shelf

Atlantic cod Gadus morhua is an extremely important fish in the northern hemisphere with respect to culture, economics, and ecology. However, the effects of over-fishing, environmental change, fish community dynamics and other factors that have altered the trophic ecology of cod are not well understood. We present an analysis of the trophic patterns of cod in the northeast US shelf ecosystem from a 25 yr time series of food habits data. Additionally, we compared the diet of this spe- cies with the spatio-temporal distribution of its prey species and evaluated prey preference over time. Atlantic cod exhibit an omnivorous diet; we assessed the temporal, spatial, and ontogenetic trends in this diet by examination of >15000 stomachs. Ontogenetic shifts in diet were observed; early juve- niles consumed more pelagic than benthic invertebrates, medium cod consumed benthic inverte- brates and fish, and larger cod consumed larger amounts of fish. Cannibalism also increased with ontogeny. Diet shifted significantly over a period of 3 decades, concurrent with changes in forage species abundance and distribution. Most of the major prey species were eaten in periods when they had high spatio-temporal overlap with cod and were abundant, indicating opportunistic feeding by cod. Similar to many other ecosystems, cod prefer sand lance, Cancer spp. crabs and herring, regard- less of the abundance or spatio-temporal overlap with these prey species. It is unclear whether the observed changes in the trophic dynamics of cod have broad implications for cod populations, yet the evidence does suggest that cod are not likely to influence the abundance and distribution of their prey populations in this ecosystem.

Does food web theory work for marine ecosystems?

More recent and extensive food web studies have questioned some of the prevailing paradigms of food web theory. Yet with few exceptions, most food webs and associated metrics are reported for freshwater or terrestrial systems. I analyzed the food web of the Northeast US Shelf ecosystem across a large spatial and temporal extent. This speciose food web exhibits a predator:prey ratio (0.95) and percentage of intermediate species (89%) similar to most other food webs. Other statistics, such as the percentage of omnivory (62%), percentage of cannibalistic species (31%), num- ber of cycles (5%), and the total number of links (L; 1562) and species (S; 81) are similar to more recent and extensively studied food webs. Finally, this food web exhibits a linkage density (L/S; 19.3), connectivity (C; 48.2%), and Lyapunov stability proxy (S × C; 39.1) that are an order of magnitude higher than other webs or are disproportionate to the number of species observed in this system. Although the exact S and C relationship is contentious, the connectivity of food webs with more than 40 species is approximately 10%, which is very different from the near 50% observed for this ecosys- tem. The openness of marine ecosystems, lack of specialists, long lifespans, and large size changes across the life histories of many marine species can collectively make marine food webs more highly connected than their terrestrial and freshwater counterparts, contrary to food web theory. Changes in connectivity also have ramifications for ecosystem functioning and Lyapunov stability. The high con- nectivity of this food web and the mathematical determinants for stability are consistent with the weak nature of species interactions that have been observed and that are required for system persis- tence. Yet the historically high exploitation rates of marine organisms obfuscate our understanding of marine food web stability. It is possible that marine food webs are inherently very different from their terrestrial or freshwater counterparts, implying the need for modified paradigms of food web theory.