Ecosystem Status Report for the Northeast Large Marine Ecosystem
5. Benthic Invertebrates
Benthic organisms primarily live on the ocean floor or within the bottom sediments. As noted earlier, these animals play an important role in energy transfer within marine systems by consuming a broad range of benthic biomass and subsequently becoming important prey items for fish and other upper trophic level animals. Benthic invertebrates such as mollusks and corals filter phytoplankton and suspended detritus from the water column, while other groups, including many crustaceans and certain species of marine worms, scavenge on the organic content of sediments and detritus that fall to the seafloor. As such, many benthic organisms serve as important conduits to couple the pelagia to the benthic habitats. Other benthic organisms such as sea stars are predators on mollusks and other benthic species. In total, over two thousand species of benthic invertebrates have been identified on the Northeast Continental shelf, although most are relatively rare. Benthic invertebrates such as lobsters, crabs, scallops, clams and sea urchins support important commercial and recreational fisheries, including some of the highest valued fisheries in the region.
Time series of population trends are available only for selected commercially and recreationally important benthic species based on directed research vessel surveys and related stock assessments. Additionally, a few ecologically important species that have been specifically sampled on NEFSC scallop surveys also have extant time series. Some of the more prominent benthic biomass trends throughout the NES LME include increases in American lobster, Homarus americanus, sea scallop, Placopectin magellanicus, and Astropecten americanus populations, and decreases in ocean quahog, Arcitca islandica, and Atlantic surfclam, Spisula solidissima, populations, especially in recent years.
The NEFSC autumn bottom trawl survey indicates that American lobster biomass has increased dramatically in the Gulf of Maine and on the Scotian Shelf, although indices for the Scotian Shelf have been highly variable in recent years (Figure 5.1a). The lobster biomass index for Georges Bank has also increased somewhat in recent years, although the increase is not of the same magnitude or consistency over time as in the Gulf of Maine. These increases are likely due to high recruitment and low commercial fishing exploitation rates (Xue et al. 2008; ASMFC 2009).
Northern shrimp, Pandalus borealis, have undergone dramatic population fluctuations in the Gulf of Maine due to variability in recruitment and regulatory changes in fishing effort (Figure 5.1b). Based on a recent assessment (ASMFC 2014), northern shrimp biomass decreased sharply in the 1990s, quickly rebounded to a high in 2006, and have decreased to historic lows in recent years, due to exceptionally low recruitment events. The period of sharp biomass increase from 2002 to 2007 was likely due to strong 2001 and 2004 year classes, as well as moderate 2003, 2005 and 2007 year classes (ASMFC 2014). Recruitment in the Gulf of Maine population is related to both spawning stock biomass and to temperature during the pelagic larval stage (Richards 2012). Colder temperatures (as seen in 2004) favor higher early life survival. Predation may be a factor in a sudden drop in biomass that occurred in 2012 (NEFSC 2014).
The NEFSC autumn bottom trawl survey biomass indices of an aggregated assemblage of crab species, including the commercially harvested deepsea red crab (Chaceon quinquedens), jonah crab (Cancer borealis), rock crab (Cancer irroratus), and lady crabs (Ovalipes spp.) have shown increases in biomass during the early 2000s, declines in all four regions in the mid 2000s, and dramatic increases in biomass for all regions except for the Scotian Shelf in recent years (Figure 5.1c). The decline in aggregated crab biomass in the mid 2000s agrees with trends from special samples taken on NEFSC scallop surveys from 2000-2010, which indicate that Cancer crab biomass in the Mid-Atlantic Bight may have declined during those years (Figure 5.1d).
Two groups of sea stars were also sampled in the Mid-Atlantic Bight on the NEFSC scallop surveys: Astropecten and Asterias (including A. forbesi, A. Vulgaris, and Leptasterias tenera). Between 2000 and 2010, it appears that Astropecten generally increased in density and expanded into shallower water on the continental shelf while Asterias densities fluctuated with lows in 2002 and 2007 to a high in 2005 (Figure 5.1d). These sea stars are known to prey on benthic invertebrates, especially bivalves, and are thought to limit the range of sea scallops to waters less than 80 m depth in the Mid-Atlantic Bight (Hart 2006). Therefore, the increases seen in Astropecten populations could impact sea scallop abundance in areas of overlap or prohibit further expansion of sea scallop distributions. Furthermore, Astropecten are generalist predators so the observed increases in density and range expansion may have unknown impacts on other benthic species and associated food webs.
Sea scallops are currently the highest valued fishery in the NES LME, and increased dramatically in biomass during the early 2000s on both Georges Bank and in the Mid-Atlantic Bight, as seen in the expanded biomass estimates from the 2014 assessment (Figure 5.2; NEFSC 2014). This dramatic increase is related to the implementation of effective management measures including reductions in fishing effort, constraints on crew size, and gear restrictions (Hart and Rago 2006). Sea scallop populations also benefitted from the establishment of long-term closed areas on Georges Bank in late 1994 and rotational closures in the Mid-Atlantic Bight. In particular, the areas where mobile fishing gear has been prohibited may have enabled a general recovery of benthos through reductions of disturbance, which may have particularly benefited sea scallops due to their relatively sessile nature. During the mid-2000s, biomass trends for sea scallops became more variable, with declines on Georges Bank during the period of increased fishing access (Figure 5.2). Since then, recruitment has improved, biomass has accumulated, and sea scallop biomass is currently at a high level on Georges Bank. However, scallops in the Mid-Atlantic Bight have declined in recent years, possibly due to predation on juveniles by Astropecten (NEFSC 2014).Recent declines in biomass for both ocean quahogs and Atlantic surfclams have been most pronounced in the southernmost region of the NES LME. Ocean quahog biomass from Southern Virginia through Long Island has declined steadily by about 46% from 1978 to 2008 (NEFSC 2009; Figure 5.2). Similarly biomass of Atlantic surfclam south of Georges Bank in a recent assessment shows a sharp decline of about 64% from 1987 to 2011, after an increase in biomass of 270% during 1979 to 1987 (Figure 5.2; NEFSC 2013). The recent declines for ocean quahog are due to low productivity and fishing, whereas the declines for Atlantic surfclam are linked to poor survival after settlement and slow growth, perhaps caused by warm water conditions and fishing (NEFSC 2013). Population estimates for ocean quahog biomass on Georges Bank, as aggregated from Georges Bank and Southern New England stocks, also indicate a slightly declining population trend over the last decade and a half (Figure 5.2), following an increase since 1978 (NEFSC 2009). Population estimates for ocean quahog biomass on Georges Bank, as aggregated from Georges Bank and Southern New England stocks, also indicate a slightly declining population trend over the last decade and a half (Figure 5.2; NEFSC 2013).
Some species are known or suspected to be important in the dynamics of the food web (Link et al. 2006) but are not routinely surveyed to assess changes in their abundance. One way to compensate for this is to use fish as 'samplers' (Link 2004; Link and Ford 2006) for groups that are understudied, under-sampled or otherwise under-estimated. We can calculate an index of relative abundance from the percent frequency of occurrence of these organisms in the stomachs of major predators in the food web. Many fish species consume food items in proportion to the abundance of the prey although other factors such as the availability of alternative prey can sometimes also be important. Examples of the representation of some important macrobenthic groups in the diets of selected fish 'samplers' are provided in Figure 5.3. The estimates derived in this way exhibit substantial variability but also demonstrate a broad coherence among the fish species used as samplers. Sea stars show increases during the late 1990s and early 2000s in a number of predators, although recent trends are variable. Sea cucumbers show declines until the 1990s, but may have increased during the mid-2000s. Sand dollars, brittle stars and sea urchins have generally varied without trend, whereas hermit crab occurrence has dramatically declined in stomachs for most predators until the early 1990s, from which point it may have stabilized at a low level (Figure 5.3). The importance of monitoring change in benthic communities is well recognized (Link and Ford 2006) and the direct and indirect sampling methods described here for benthos of the NES LME meet a critical need.