It should be no secret to readers of this blog or anyone familiar with fisheries at all that spiny dogfish have a pretty rotten reputation among commercial fishermen.  Quickly approaching the notoriety of dogfish are those damn dirty sea mammals, the harbor seals.  Both animals have enjoyed recent increases in population: dogfish from a period of proactive fishery management, and seals from riding along on the Marine Mammal Protection Act.  Arguments against protections for both of these species usually hinge on the alleged economic damage they do to fisheries by stealing the catch, destroying gear, and consuming commercially-important species.  However, until recently there hasn’t been much concrete information on just how damaging these “pests” can be.  A new paper by researchers working in the Cape Cod gillnet fishery attempts to answer that question, at least for New England groundfish.

One of the biggest issues in fisheries management is discarded catch.  Discards are frustrating for fishermen and managers alike because they represent fish that were likely wasted after being caught in the gear.  Most discards result from regulations (think size limits, prohibited species, etc.), but some are perfectly marketable fish that have to be tossed because they’ve been partially eaten.  Gillnets are especially problematic for scavenging because the captured fish are immobile in the net until they’re brought up by the fishermen, and present an irresistible target for scavengers (just look at what happened to some fish we caught when the hagfish got to them).  Dogfish and seals also present unique issues if they get caught in the process of scavenging: dogfish have an amazing ability to totally wrap themselves up in the net, and accidentally drowning a seal can get a fisherman in a boatload of trouble.

Rafferty et al. (2012) don’t really address the capture issue with scavenging dogfish and seals, and instead focus on the direct impact of the actual scavenging.  With plenty of other nibblers out there aside from dogfish and harbor seals, it’s important to be able to separate which scavenger is at fault.  Fortunately, both spiny dogfish and harbor seals have very distinctive bites.

The signature bite patterns of a.) harbor seals and b.) spiny dogfish. From Rafferty et al. (2012).

Seals tend to like to tear out the juicy bits of their fish prey, taking out the innards (especially the liver) and often leaving the rest of the fish behind.  Dogfish, on the other hand, are there for the delicious fishmeat, and leave clean bites in the fillet areas (or skeletonize the fish altogether).  Either way, a fisherman won’t be able to sell fish that damaged.

Once it was possible to identify how much damage to could be attributed to which scavenger, it was a simple matter for Rafferty et al. (2012) to put a monetary value to it.  In their sampling period they found that dogfish accounted for 1.98% of the catch and seal munched 0.40%, for a total of 2.38% of the catch discarded due to scavenging damage.  The total catch observed by Rafferty et al. (2012) during their sampling period was worth $61,800, and they estimated that dogfish and seals ate $2,250 worth of it.  The paper concludes that at the current market price for haddock (at the time of the study), scavenging by these to animals would cost fishermen 3.64% of the market value of their catch.  This may not seem like much, but Rafferty et al. (2012) do note that the populations of both scavengers are continuing to increase, so this represents a baseline estimate.

While dogfish and seal scavenging is certainly annoying to deal with (and Rafferty et al. (2012) never even touch on the joys of removing these animals from the net) it seems to pale in comparison to other issues facing fishermen when it comes to making a decent buck.  That said, every little bit helps.  The paper recommends shorter soak times to allow scavengers less time to get at the catch, but this may carry its own pitfalls as longer soak times usually mean a bigger catch (even if not all of it is in the best shape).  Shorter soak times do carry the benefit of a lower mortality rate for the scavengers.  Ultimately, both dogfish and seals have been hunting and scavenging fish for far longer than humans have even been around, so it’s up to us to adapt to them.

RAFFERTY, A., BRAZER, E., & REINA, R. (2012). Depredation by harbor seal and spiny dogfish in a Georges Bank gillnet fishery Fisheries Management and Ecology DOI: 10.1111/j.1365-2400.2011.00837.x