Community size spectra provide indicators of ecosystem recovery on the Newfoundland and Labrador shelf

Abstract

Species- and size-selective overexploitation often have ecosystem-wide impacts that are evident in community size spectra. To both derive potential ecosystem targets for community rebuilding and assess contemporary indicators relative to these targets, we constructed theoretical size spectra to predict pristine biomass densities using a combination of species- and size-specific nitrogen stable isotope signatures and a range of trophic efficiencies and primary productivity estimates within and among 3 sub-regions of the Newfoundland and Labrador shelf. Theoretical size spectra were compared to empirically derived size spectra using trawl survey data. The descending slopes of the empirically determined size spectra were between 1.25 and 2.42 times steeper than the theoretical slopes. The percentage of the theoretical distribution represented by the empirical size structure ranged between 1.1 and 29.4%, with the closest and furthest estimates associated with the smallest- and largest-sized fishes, respectively, and strongly influenced by estimates of trophic efficiency and primary productivity. Regional variation was also observed, with southern regions reaching 1.3-32.3% of the theoretical biomass density and the northernmost region reaching only 0-8.3%. Importantly, the descending slopes varied depending on fish size, with biomass density of larger sizes decreasing faster than that of smaller sizes. Variations among sub-regions and fish guilds were also observed. These analyses provide a means to derive potential ecosystem targets and indicators through which recovery of fish communities can be monitored and assessed.

Spatial variation in food web structure in a recovering marine ecosystem

“…compared to the other two regions [6]. Given both this hypothesis and our reported patterns, further examinations of benthic-pelagic prey contributions to predator diets should be included in analyses of differential recovery within and among marine ecosystems.…”

“…Three abundant fish species of socioeconomic importance were analyzed in greater detail due to recent debates regarding their population status and interactions with other fisheries. The biomass of these predators is often greater in the southern region studies than the north [116][117][118][119] and represented the most abundant species caught in our bottom trawls [6,47]. We observed that shrimp and/or crab often comprised a substantial portion of the diet with the proportion increasing with ontogeny (Figs 4-6).…”

“…We report the results of within-and among-region variation in the Newfoundland and Labrador shelf marine ecosystem analyzed using isotope mixing models to examine whether regional variation in community overlap [47] and differential size-based community recovery rates [6] may be explained by differences in energy and nutrient flows through ecosystems recovering from overexploitation [5]. While every food web is inherently a simplification of reality, based on the results of both the stomach content and the stable isotope analyses, the importance of specific forage species changes markedly among the three analyzed regions.…”

“…One impact of this change was interactions among species through predation and indirect effects [4]. Now, more than two decades after stock collapses and initial fisheries moratoria, the ecosystem has not fully recovered [5,6]. Factors hypothesized to influence population trajectories include continued fishing [7], depensatory growth [8], climate change [9], and life history changes [10].…”

“…Factors hypothesized to influence population trajectories include continued fishing [7], depensatory growth [8], climate change [9], and life history changes [10]. However, Newfoundland and Labrador marine ecosystems also exhibit spatial heterogeneity in their structure and function with northern communities displaying lower fish species diversity and diet diversity [6,11], and lower recovery of marine fish community size-structure [6]. In an ecosystem context, it is important to quantify the extent to which spatial variability in diets on relevant timescales may also contribute to differing population and community trajectories.…”