What Drives Dogfish Distribution?

I’ve been meaning to get back into writing about research done by other people (especially if it concerns the mighty spiny dogfish) for some time.  Fortunately, a pretty interesting paper has just come out by researchers at Stony Brook and the National Marine Fisheries Service (NMFS) looking at the big picture of spiny dogfish habitat preferences.  So in what conditions can you expect to run into those big, lovable, bait-stealing schools?

Sagarese et al. (2014) looked at dogfish catches from NMFS spring and autumn bottom trawl surveys going from 1963-2009, an absolutely massive data set.  Though there has been some controversy over the effectiveness of NMFS trawl survey methods, this is still an extremely long-running, standardized data set with mountains of information on fish species along the U.S. east coast, and one of the best in fisheries science.

Here’s what the overall results look like in map form, and there are some striking trends already visible.

Spiny dogfish catches from 1963-2009 NMFs spring bottom trawl surveys. The darker the dot, the higher the catch. Pie charts show total dogfish catch by region (white=Mid-Atlantic, light grey=Southern New England, grey=Georges Bank, dark grey=Gulf of Maine) (a=1960s, b=1970s, c=1980s, d=1990s, e=2000s). From Sagarese et al. (2014).

Spiny dogfish catches from 1963-2009 NMFs spring bottom trawl surveys. The darker the dot, the higher the catch. Pie charts show total dogfish catch by region (white=Mid-Atlantic, light grey=Southern New England, grey=Georges Bank, dark grey=Gulf of Maine) (a=1960s, b=1970s, c=1980s, d=1990s, e=2000s). From Sagarese et al. (2014).

On every map except the one representing the 1960s (when only offshore sites were towed), you can make out high catches along both the shoreline and shelf break with relatively low catches in between.  Also, the distribution seems to have changed quite a bit in the past decade.  Nearly half of captured dogfish were caught in Southern New England through the 1970s, then dogfish were about evenly distributed between all the regions except he Gulf of Maine in the 1980s-90s.  Since then, dogfish seem to have really taken a liking to the Mid-Atlantic region (encompassing the waters between New York and Cape Hatteras), with 62% of captured dogfish showing up in this patch of water.

So what drives the distribution of dogfish?  Sagarese et al. (2014) sought to answer that by digging into the environmental data taken at each trawl site and breaking the dogfish catch into five groups: mature females, mature males, immature females, immature males, and neonates.  They then determined whether depth, temperature, salinity, and latitude significantly influenced catches for each dogfish group, and compared those conditions between the groups.  What they found reveals quite a bit about dogfish distribution, and may have implications for making the dogfish fishery more sustainable.

Dogfish aggregating, probably in response to some environmental variable. Photo by Andy Murch (elasmodiver.com)

What the researchers found was that each dogfish demographic had it’s own set of environmental preferences, and that these could vary by season.  Generally, younger dogfish preferred sites that were significantly warmer, more saline, and deeper than mature sharks, and mature females were generally found at considerably shallower depths than males or immature sharks.  Differences in latitude were more significant in spring, with mature females occupying more southern waters than the other dogfish groups.

These findings are interesting for a few reasons.  First, the life cycle of spiny dogfish is almost the exact opposite of the life cycle of most other coastal sharks.  Most sharks inhabiting coastal habitats are born in either close to the beach or within estuaries, and then move farther offshore as they grow.  Spiny dogfish appear to use the shelf break as a nursery area, growing up in very deep water and moving closer to shore as they grow.  Most of the spiny dogfish’s closest relatives are deepwater species, so does this behavior represent an evolutionary throwback?  This behavior also effectively puts their nursery habitat out of the reach of most U.S. fisheries and just barely within reach of NMFS surveys, which may have had something to do with the species’ unexpectedly speedy recovery from overfishing.

Perhaps most importantly, with such clear environmental boundaries between dogfish life stages, is it possible for the dogfish fishery to target less-vulnerable mature males instead of females?  With the current market price for dogfish it may not be economically viable for fishermen to chase them out to the shelf break, but if the fishery expands this may be the future of dogfishing.

References

Sagarese, S., Frisk, M., Miller, T., Sosebee, K., Musick, J., & Rago, P. (2014). Influence of environmental, spatial, and ontogenetic variables on habitat selection and management of spiny dogfish in the Northeast (US) shelf large marine ecosystem Canadian Journal of Fisheries and Aquatic Sciences, 1-14 DOI: 10.1139/cjfas-2013-0259

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