Jonathan A. D. Fisher

Transient dynamics of an altered large marine ecosystem

Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada’s east coast and elsewhere resulted in restructuring of entire food webs now dominated by planktivorous, forage fish species and macroinvertebrates. Despite the imposition of strict management measures in force since the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure. Here we provide evidence of the transient nature of this ecosystem and its current return path towards benthic fish species domination. The prolonged duration of the altered food web, and its current recovery, was and is being governed by the oscillatory, runaway consumption dynamics of the forage fish complex. These erupting forage species, which reached biomass levels 900% greater than those prevalent during the pre-collapse years of large benthic predators, are now in decline, having outstripped their zooplankton food supply. This dampening, and the associated reduction in the intensity of predation, was accompanied by lagged increases in species abundances at both lower and higher trophic levels, first witnessed in zooplankton and then in large-bodied predators, all consistent with a return towards the earlier ecosystem structure. We conclude that the reversibility of perturbed ecosystems can occur and that this bodes well for other collapsed fisheries.

Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem

Globally, overfishing large-bodied groundfish populations has resulted in substantial increases in their prey populations. Where it has been examined, the effects of overfishing have cascaded down the food chain. In an intensively fished area on the western Scotian Shelf, Northwest Atlantic, the biomass of prey species increased exponentially (doubling time of 11 years) even though the aggregate biomass of their predators remained stable over 38 years. Concomitant reductions in herbivorous zooplankton and increases in phytoplankton were also evident. This anomalous trophic pattern led us to examine how declines in predator body size (approx. 60% in body mass since the early 1970s) and climatic regime influenced lower trophic levels. The increase in prey biomass was associated primarily with declines in predator body size and secondarily to an increase in stratification. Sea surface temperature and predator biomass had no influence. A regression model explained 65 per cent of prey biomass variability. Trait-mediated effects, namely a reduction in predator size, resulted in a weakening of top predation pressure. Increased stratification may have enhanced growing conditions for prey fish. Size-selective harvesting under changing climatic conditions initiated a trophic restructuring of the food chain, the effects of which may have influenced three trophic levels.

From Sea to Sea: Canada's Three Oceans of Biodiversity

Evaluating and understanding biodiversity in marine ecosystems are both necessary and challenging for conservation. This paper compiles and summarizes current knowledge of the diversity of marine taxa in Canadas three oceans while recognizing that this compilation is incomplete and will change in the future. That Canada has the longest coastline in the world and incorporates distinctly different biogeographic provinces and ecoregions (e.g., temperate through ice-covered areas) constrains this analysis. The taxonomic groups presented here include microbes, phytoplankton, macroalgae, zooplankton, benthic infauna, fishes, and marine mammals. The minimum number of species or taxa compiled here is 15,988 for the three Canadian oceans. However, this number clearly underestimates in several ways the total number of taxa present. First, there are significant gaps in the published literature. Second, the diversity of many habitats has not been compiled for all taxonomic groups (e.g., intertidal rocky shores, deep sea), and data compilations are based on short-term, directed research programs or longer-term monitoring activities with limited spatial resolution. Third, the biodiversity of large organisms is well known, but this is not true of smaller organisms. Finally, the greatest constraint on this summary is the willingness and capacity of those who collected the data to make it available to those interested in biodiversity meta-analyses. Confirmation of identities and intercomparison of studies are also constrained by the disturbing rate of decline in the number of taxonomists and systematists specializing on marine taxa in Canada. This decline is mostly the result of retirements of current specialists and to a lack of training and employment opportunities for new ones. Considering the difficulties encountered in compiling an overview of biogeographic data and the diversity of species or taxa in Canadas three oceans, this synthesis is intended to serve as a biodiversity baseline for a new program on marine biodiversity, the Canadian Healthy Ocean Network. A major effort needs to be undertaken to establish a complete baseline of Canadian marine biodiversity of all taxonomic groups, especially if we are to understand and conserve this part of Canadas natural heritage.

Breaking Bergmann's rule: truncation of Northwest Atlantic marine fish body sizes

A strictly species-centric view of human impacts on ecological communities may conceal important trait changes key to ecosystem functioning and stability. Analyses of body size and community composition data for 326 Northwest Atlantic fish species sampled across > 900000 km2 over three decades revealed a rapid and widespread reduction of body sizes driven by declines within species and changes in relative abundances. The changes were unrelated to species richness but of sufficient magnitude to eliminate biogeographic scale gradients of increasing body size with latitude commonly characterized as Bergmanns rule. These changes have persisted despite reduced potential for intraspecific competition and favorable bottom water temperatures, both of which should lead to increased growth rates. The aggregate body sizes in these Northwest Atlantic fish communities may now represent a mismatch between the environmental variability characteristic of the Northwest Atlantic and the historical body size, life history traits, and productivity of species across this region. We discuss how these changes may jeopardize the potential for recovery of these important temperate/subarctic ecosystems.

Temporal dynamics within a contemporary latitudinal diversity gradient

Poleward declines in species diversity [latitudinal diversity gradients (LDG)] remain among the oldest and most widespread of macroecological patterns. However, their contemporary dynamics remain largely unexplored even though changing ecological conditions, including global change, may modify LDG and their respective ecosystems. Here, we examine temporal variation within a temperate Northwest Atlantic LDG using 31 years of annual fisheries-independent surveys and explore its dynamics in relation to a dominant climate signal [the wintertime North Atlantic Oscillation (NAO)] that varies interannually and alters the latitudinal gradient of Northwest Atlantic continental shelf bottom water temperatures. We found that the slopes of the annual LDG vary dramatically due to changes in geographic distributions of 100+ species, variations that are concealed within the cumulative, static LDG. These changes are strongly associated with changes in NAO sign and strength. This is the first illustration of temporal dynamics in a contemporary LDG and the first demonstration of the speed at which local environmental variations can alter an LDG. Our findings underscore the need to investigate factors that modify LDG separately from those that contribute to their origins.

Migration patterns and putative spawning habitats of Atlantic halibut (Hippoglossus hippoglossus) in the Gulf of St. Lawrence revealed by geolocation of pop-up satellite archival tags

Arnault Le Bris,* Jonathan A. D. Fisher, Hannah M. Murphy, Peter S. Galbraith, Martin Castonguay, Timothy Loher, and Dominique Robert Centre for Fisheries Ecosystems Research, Fisheries and Marine Institute, Memorial University of Newfoundland, P.O. Box 4920, St. John’s, NL A1C 5R3, Canada Institut Maurice-Lamontagne, Pêches et Océans Canada, CP 1000, Mont-Joli, QC G5H 3Z4, Canada International Pacific Halibut Commission, 2320 West Commodore Way, Suite 300, Seattle, WA 98199-1287, USA *Corresponding author: tel: þ1 709 778 0482; fax þ1 709 778 0669; e-mail: arnault.lebris@mi.mun.ca. Present address: Northwest Atlantic Fisheries Centre, Fisheries and Oceans Canada, 80 East White Hills Road, PO Box 5667, St. John’s, NL A1C 5X1, Canada. Present address: Canada Research Chair in Fisheries Ecology, Institut des Sciences de la mer, Université du Québec a Rimouski, 310 allée des Ursulines, P.O. Box 3300, Rimouski, QC G5L 3A1, Canada.

Characterization of Depth Distributions, Temperature Associations, and Seasonal Migrations of Atlantic Halibut in the Gulf of St. Lawrence using Pop-Up Satellite Archival Tags

AbstractThe fishery for Atlantic Halibut Hippoglossus hippoglossus in the Gulf of St. Lawrence (Gulf) is currently experiencing its highest landings since the 1950s. However, the absence of information on adult habitat use has limited the development of new survey methodologies. The aim of this study was to use pop-up satellite archival tags on large (≥108-cm) halibut in the Gulf to provide data on seasonal temperature associations, depth distributions, and migrations. Twenty Atlantic Halibut were tagged in 2013 and 15 were tagged in 2015 at two different locations in the northern Gulf. Atlantic Halibut overwintered in the central and northern Gulf based on six tag pop-offs. In the winter in both studies, halibut were distributed at 160–440 m depth with a narrow temperature association of 5.5–6.5°C, which corresponded with the bathymetry and hydrography of the Gulf rather than the deeper and colder waters of the continental shelf where the southern stock occurs. Spawning rises were identified from the dep...

First estimates of Greenland shark ( Somniosus microcephalus ) local abundances in Arctic waters

Baited remote underwater video cameras were deployed in the Eastern Canadian Arctic, for the purpose of estimating local densities of the long-lived Greenland shark within five deep-water, data-poor regions of interest for fisheries development and marine conservation in Nunavut, Canada. A total of 31 camera deployments occurred between July-September in 2015 and 2016 during joint exploratory fishing and scientific cruises. Greenland sharks appeared at 80% of deployments. A total of 142 individuals were identified and no individuals were observed in more than one deployment. Estimates of Greenland shark abundance and biomass were calculated from averaged times of first arrival, video-derived swimming speed and length data, and local current speed estimates. Density estimates varied 1–15 fold among regions; being highest in warmer (>0 °C), deeper areas and lowest in shallow, sub-zero temperature regions. These baited camera results illustrate the ubiquity of this elusive species and suggest that Nunavut’s Lancaster Sound eco-zone may be of particular importance for Greenland shark, a potentially vulnerable Arctic species.

Global Aquatic Ecosystem Services Provided and Impacted by Fisheries: A Macroecological Perspective

Two and a half decades ago, macroecology was initially characterized as the study of the division of food and space among species on continents. Since then, macroecological research has been expanded to include the study of relationships between organisms and their environments by characterizing and explaining patterns of abundance, distribution, and diversity. Importantly, macroecology was meant to counter what was seen as increasingly reductionist and specialized approaches in ecology and to examine the roles and interactions among ecological and evolutionary explanations for large-scale patterns using integrated analyses. Macroecological investigations occur at spatial scales increasingly relevant to biodiversity and ecosystem functioning research, global change dynamics, and fisheries. This chapter highlights patterns and interactions among key macroecological variables within aquatic ecosystems, identifies approaches to overcome the challenge of identifying processes underlying macroecological patterns, reviews the emerging traits-based focus on aquatic functional diversity, and describes ecological and evolutionary effects of selective fisheries on aquatic ecosystem functioning.