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Another Cod Spawning Ground Confirmed Through Acoustic and Trawl Surveys

Jun 19, 2017
2017 Issue 2

During the winter of 2015/2016, GMRI scientists collaborated with industry to study a population of cod that spawn during the late fall/winter (hereafter referred to as winter) in a small area off Ipswich Bay known as The Cove. Beginning in 2014, the New England Fisheries Management Council had prioritized research that would increase understanding of groundfish spawning on Georges Bank and in the Gulf of Maine. GMRI was awarded funds by the New England Fishery Management Council to undertake research on cod that spawn in the Cove during the winter, an understudied spawning group.

The resultant research yielded two important findings:

  • The study has confirmed that a considerable biomass of Atlantic cod (at least 250 metric tons) aggregate and spawn in The Cove during  November and December; and
  • The survey technique that GMRI employed holds promise as a complementary method for estimating the abundance of cod in the Gulf of Maine.

Ipswich Bay Winter Spawning population:

GMRI worked with fishermen to sample the cod population in The Cove using both standard groundfish gear and scientific-grade echo sounders (i.e., acoustics). The trawl samples were used to determine size, age, sex, maturity stage, and relative condition of individuals in the sample (i.e., demographics). Trawl data was also used to calibrate and verify acoustic signals. Data from both sources generated a rich picture of the abundance, demographics, and distribution of the spawning population in both space and time, since the area was surveyed a total of 6 times over the course of 3 months. Trawl data identified a peak in the catch-per-unit-effort and proportion of spawning cod in early to mid-December. Acoustic data revealed that cod were most abundant in mid- to late November suggesting that the cod aggregate in The Cove before reaching peak spawning condition.

With this new data, The Cove joins 3 other well-studied spawning locations in the western Gulf of Maine: The Gulf of Maine Cod Spawning Protection Area (a spring spawning location also in Ipswich Bay), the Spring Cod Conservation Zone in Massachusetts Bay, and the Winter Cod Conservation Zone in Massachusetts Bay.

Implications of new survey technique:

Sampling a spawning aggregation using trawl and acoustic gear can offer new information to supplement more traditional trawl survey techniques. By sampling an area during a known aggregation event, we can potentially establish a baseline biomass estimate that could complement trawl survey results. Our low-end acoustic-based estimate of the spawning population in The Cove is 250 metric tons (mt), which would represent 11% of the total Gulf of Maine biomass, indicated by the 2014 stock assessment. Even though it is unlikely that we will ever fully describe all spawning locations in the Gulf of Maine, if we add up acoustic-based estimates of biomass from the known spawning locations, when cod are highly aggregated, we may be able to produce minimum estimates of biomass that could complement standard trawl survey-based estimates.

Implications for management:

Management of cod in the Gulf of Maine has traditionally recognized the need to protect spawning aggregations. The rolling closures in the western Gulf of Maine reflect this approach. Our results provide new data for the council to consider when reviewing and revising spawning closures. Current strategies are based on 10-minute squares, which can be closed for entire months. Our survey approach, if applied to other spawning areas, could result in shorter closures of smaller areas. 

Note: This is an updated version of an article that originally appeared in Waypoints on 6/19/2017.