Ecosystem Status Report for the Northeast Large Marine Ecosystem
6. Fish Communities
Here we provide a community-level perspective on changes in fish populations over time to complement existing single species-based approaches. We focus on species groups aggregated in different ways including taxonomic affinities, fishing practices, habitat, and diet or trophic level. Integrative measures can provide insights into how an ecosystem responds to a wide range of pressures and drivers. This “big picture” view of the biotic components of the food web provides a better sense of relationships among its component species and processes of energy flow within this ecosystem.
Often, broadly defined taxonomic groups are targeted by different components of the fishing fleet. Here we consider four such groups including groundfish (the traditional target species for the bottom-trawl and gillnet fleets), small elasmobranchs (skates and dogfish), small pelagic fishes (principally Atlantic herring and Atlantic mackerel), and other fish. The small elasmobranchs were primarily caught incidentally in groundfish fisheries until markets for these species were further developed in the 1980’s. The small pelagic fishes have long supported important commercial fisheries (including some of the oldest in the United States) and are often targeted using specialized gear. Many of the species included in the other fish category are taken as incidental catch but some, notably monkfish, have emerged as extremely valuable components of the overall fishery.
Catch per tow by functional group
An examination of trends in each of these groups over the entire shelf region based on NEFSC autumn bottom trawl surveys points to dramatic increases over time in the small elasmobranch and pelagic fish components. In contrast, an initial decline and subsequent recovery is evident for the groundfish category, while other fish have remained stable or increased (Figure 6.1). These patterns are related to harvesting practices that resulted in sharp declines in bottom-dwelling fish, and subsequent implementation of management measures in the 1990s which resulted in recovery of at least some of the groundfish species.
Total biomass of all functional groups by ecological production unit
Total biomass can be estimated using all functional groups for each of the four ecological production units. Partitioning total biomass this way shows differences among the regions (Figure 6.2). Fall biomass on Georges Bank has gradually increased over time while the biomass in the Gulf of Maine has increased rapidly in the past decade. The other two regions have been relatively more consistent with biomass in the Mid-Atlantic Bight decreasing in recent years. Biomass levels in the Scotian Shelf are generally less than the other three regions.
Pelagic to Demersal ratio by ecological production unit
The species comprising a community and those targeted by a fishery can be characterized by how they are partitioned into different habitats. For example, demersal fish species such as cod, haddock, and flounders are found in near-bottom waters or are associated with the seabed. In contrast, pelagic fish species such as herring and mackerel are typically found higher in the water column. An index of the ratio of pelagic to demersal fishes provides important insights into relative changes in these two major groups of fishes and, therefore, pathways of energy flow (Link 2005; Methratta & Link 2006). In the early part of the time series, the demersals were dominant throughout the northeast continental shelf (aside from the Mid-Atlantic Bight) as shown by low ratios across three regions. Starting in the late 1980s, a shift occurred from a demersal dominated system to a pelagic dominated one. This shift occurred later in the Gulf of Maine. In contrast, the Mid-Atlantic was generally dominanted by pelagics (Fogarty & Murawski 1998; Link et al. 2011; Figure 6.3). Recently the trend of increasing pelagic to demersal ratios has leveled off and even reversed on the Scotian Shelf. Conversely, the Mid-Atlantic is now dominated by demersal species albeit near the 1:1 mark.
Biodiversity, the mix of species within the ecosystem, provides a range of ecological services and increases the stability of ecosystem functions (Cardinale et al. 2012). It is therefore a secondary goal of fisheries management. However, this can be difficult to do with traditional single species management but can be addressed more explicitly within an EBFM context. There are a number of different metrics that can be used to measure the biodiversity of an area. One of the more robust metrics is Hurlbert’s expected number of species which standardizes the sample size between tows (Hurlbert 1971). This is necessary because the more individuals within a catch, the higher the probability of encountering a rare species. Changes in biodiversity can be attributed directly to fishing but also indirectly to other human impacts. It may be impossible to fully comprehend the human impact on biodiversity. For the NES LME, trends in the expended number of species follow one of two patterns; expected numbers either increased during the middle of the time series with recent slight declines or vice versa (Figure 6.4). The biggest change in biodiversity has been in the Mid-Atlantic which went from around 9.5 species to a low of 6.0 species before rebounding to levels similar to the beginning of the time series.
We can also characterize the fish community and species of commercial and ecological significance with respect to their average size (Link 2005, Methratta & Link 2006). An indicator of overall mean length, taken from the lengths of all finfish species caught in fishery-independent surveys, reflects changes in the size composition of the entire fish community. Georges Bank has remained relatively stable in terms of the average fish size over four decades, with some evidence of a slight recent decline (Figure 6.5).
Decreases in average size are evident in the other ecological production units but with different timing in the onset of the declines (Figure 6.5). The decrease in mean size has been relatively continuous in the Gulf of Maine since the inception of the NEFSC surveys. In contrast, the Scotian Shelf exhibited an increase until the early 1990s after which sharp declines were evident. The Mid-Atlantic region showed initial declines followed by a stabilization at low mean size, with a recent increase in this area. How these general patterns may change as fishes continue to shift their distribution (Nye et al. 2009) will remain an intriguing factor to evaluate.
The “trophic level” (TL) of a species (its place in the food web) is an important aspect of understanding not only the implied size of species in an ecosystem, but also the transfer of energy in the system (Pauly et al. 1998). We can determine the trophic level of a species from examining its diet. It is then possible to determine the mean trophic level in the sampled fish community by weighting the trophic levels of individual species by their abundance (biomass) and averaging over all species. The mean trophic level is an indicator of how much energy is transferred to species feeding higher up in the food web. The mean trophic level of fish species captured during the NEFSC autumn bottom trawl surveys has remained relatively stable over time for three of the four ecological production units (Figure 6.6). The mean trophic level of the Scotian Shelf has declined in recent years while the Mid-Atlantic declined through the early 90s, rebounded, and is experiencing another decline.
We note that declines in condition factor, or individual fish weight in relation to fish length, have been observed for numerous fish stocks in the Northeast US. Recently, trends in condition factor were analyzed for 40 finfish stocks caught in the NEFSC autumn bottom trawl survey (1992-2010), and sexes were analyzed separately for species whose growth rate differ by sex. Most of fish stocks and sexes (47 of the 66 combinations) were found to have significant trends in condition factor over the time series, and of these, only 5 showed a significant increase in condition factor (Northern silver hake females, Southern silver hake males, both sexes of Northern windowpane flounder, and Southern windowpane flounder females) (Figure 6.7). Changes in condition factor can be due to fishing pressure, competition, or environmental changes. However, further analysis showed that abundance or bottom temperatures did not appear to be driving the observed decreases in fish weight. Similar changes in condition have been noted for fish in Atlantic Canada. The overall change in fish condition is important because the productivity of fish stocks and expected yield depend on growth and condition. Further, the reproductive output of fish stocks is linked to their condition, potentially affecting egg production and recruitment.
Estimates of groundfish recruitment (the number of young fish surviving to a specified age) and the overall weight of the adult population producing this recruitment since 1985 are available for fourteen stocks on the Northeast Continental Shelf. We examined the estimated number of recruits divided by the spawning stock that produced this recruitment. To compare this metric for all stocks together, we standardized each ratio relative to its average value and its standard deviation (a measure of the variability of the ratio). We do see periods of apparently favorable and unfavorable recruitment levels (Figure 6.8). Many of these stocks had poor recruitment index values during 2001-2003 and 2008-2009 in particular. In addition to environmental drivers that can affect the recruitment survival index, it is possible that other factors such changes in age-structure of the stocks and related changes in egg and larval survival can come into play. There is increasing evidence that the viability of the progeny from older and larger female spawners is higher.