Does the flourishing population of spiny dogfish have an impact on popular bottom fish like sea bass and flounder species? Research findings by NOAA Fisheries’ Northeast Fisheries Science Center (NFSC) published in Ecology and Evolution reveal rates of interactions between cod and spiny dogfish are higher than previously thought, so it’s more than possible and perhaps even likely.
In the study published in late 2020, researchers examined the stomach contents of 295 dogfish samples collected on 15 fishing trips during normal trawling operations between May 2014 and May 2015 in the Gulf of Maine and on Georges Bank. Using the conventional visual method, they observed 51 different prey types and nearly 1600 individual prey items. NOAA Fisheries scientists paired these visual observations with a laboratory technique (real-time polymerase chain reaction, or PCR) to detect small amounts of cod DNA. Using this technique, researchers examined 291 of the 295 available samples and detected cod DNA in 31 of them.
“This was an excellent example of how cooperating fishing partners supplied fish for a pilot study of interest, and have helped advance this field of study,” said Richard McBride, chief of the NFSC’s Population Biology Branch and a co-author of the study. “We were able to demonstrate that identifying cod in predator stomachs with environmental DNA works. It let us show fishermen that these innovative laboratory techniques can work on samples collected in the open ocean.”
According to NOAA Fisheries, dogfish primarily eat other fish, but also jellyfish, squid and bivalves in some locations. Cod as dogfish prey is rare, with only 14 cod having been visually observed in the stomachs of 72,241 dogfish collected by the science center’s bottom trawl surveys from 1977 to 2017. This suggests low predation rates on cod. However, small cod are much more likely to be well-digested when the samples are taken. If dogfish have eaten these smaller cod, it is difficult to identify the species by observation alone.
Molecular-level studies, using DNA, offered some answers. While the findings suggest higher interaction rates between dogfish and cod than previously observed, further study is needed to determine just how much cod dogfish eat. Researchers say the next step is to use a statistically robust sampling design to examine a population-level assessment of the effects of dogfish predation on cod population size. Estimates of spiny dogfish digestion rates, and ways to consider dogfish scavenging during fishing operations, are also needed.
NOAA Fisheries declared spiny dogfish as overfished in 1998 with a federal fisheries plan fully implemented by the beginning of the 2000–2001 fishing year. The resulting cutbacks in commercial harvest of spiny dogfish in the 2000’s effectively collapsed the overseas market; by the time spiny dogfish was declared rebuilt in 2009, U.S. commercial fishermen essentially had no one to buy their catch any longer resulting in a population boom.
Anecdotal observations by commercial and recreational fishermen over the years have been soundly rebuffed by environmentalists like Sonja Fordham, a shark specialist with the International Union for Conservation of Nature (IUCN). “I think it’s a very popular notion to say this voracious predator is scarfing up all the good stuff,” Fordham said back in 2009, adding “The facts don’t back it up.”
Fordham and the IUCN were instrumental in shutting down the commercial market for spiny dogfish, calling the despised predator a “perfect scapegoat” for the fishery’s problems, instead blaming decades of overfishing for the downtrend in Atlantic cod.
In response to Fordham’s activism on behalf of spiny dogfish, an ad hoc group of recreational, commercial and party/charter fishing associations was formed in the 2000’s calling itself Fishermen Organized For Responsible Dogfish Management, or FORDM, an acronym meant as a jab at Fordham.
In a 2009 letter to then director of NOAA Fisheries, Jane Lubchenco, FORDM said “The depredations of this huge biomass of spiny dogfish on other, competing species (or on the forage that those species are dependent upon) are interfering significantly with the rebuilding of those species, and we – the fishermen – are paying for that through lowered recreational and commercial quotas.”
The ad hoc association went on to quote a 1953 study by researchers Henry Bigelow and William Schroeder (Fishes of the Gulf of Maine), “Voracious almost beyond belief, the dogfish entirely deserves its bad reputation. Not only does it harry and drive off mackerel, herring, and even fish as large as cod and haddock, but it destroys vast numbers of them.”