Cusk (Brosme brosme) COSEWIC assessment and status report 2012: chapter 11

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Threats and Limiting Factors

Overfishing is the most important threat to Cusk. There are two sources of information on the impact of fishing on the Cusk population. The first is the landings reported by NAFO statistical area on an annual basis and the second is discards in various fisheries as reported by various observer programs. Regarding landings, most Cusk is taken as bycatch in fisheries directed at other species, particularly Cod, Haddock and Pollock. Traditionally, a longline fishery for 'shack' has been recognized, which was directed for Cusk and White Hake combined. 'Shack fishing' (Cusk and White Hake directed fishing combined) accounted for 50% of the Cusk catch prior to 1999 and 33% subsequently (COSEWIC 2003).

The nominal catch ^[2] of Cusk in the Northwest Atlantic Fisheries Organization (NAFO) Statistical Areas (Figure 35, Table 15) varied between about 3000t and 8000t from 1960, when records began, until the early 1990s, at which time landings fell to less than 2000t. Almost all landings came from the Scotian Shelf – Gulf of Maine area. Small quantities were also caught on the Grand Banks (SA3) and, in the 1960s and 1970s, off West Greenland (SA1). Almost no landings were reported from off Labrador (SA2) or from the Gulf of St. Lawrence – Sydney Bight (4RSTVn).

Figure 35. Total annual reported landings of Cusk in the Northwest Atlantic.

While US landings were significant in the 1980s, the majority of landings have been reported by Canada, this by vessels operating in DFO’s Maritimes Region. The Canadian share of the landings from 1960 to present has been 78% while the US share has been 20%. The remainder is accounted for by various other countries, predominantly fishing in the northern areas (SA1-3).

Canadian landings have predominantly been from the western Scotian Shelf – Bay of Fundy (4X) with Georges Bank (5Z^[3]) being second in importance (Table 16). Total Canadian catches were as high as 6000 t in a number of years but have been consistently below 2000 t since 1995. The USA fishery was limited to NAFO Subarea 5 after the international boundary determination in 1984. Prior to that date, less than 5% of USA catches were reported from SA4. Within SA5, about 2/3 of USA catches were reported as being from the Gulf of Maine (5Y) and 1/3 from Georges Bank. Total USA catches were as high as 2000 t in some years but have been less than 500 t since 1995 (Table 17).

Since the 1970s, the Canadian fishery has been dominated by longline vessels (Table 18) operating during the second and third quarter of the year (Figure 36).

Figure 36. Longline landings of Cusk by quarter of the year.

Regarding the second source of mortality (discards), there are no data available from ongoing monitoring of the fishery. There are, however, recent studies which allow estimation of Cusk discard rates. Regarding post-capture mortality (PCM), Harris and Hanke (2010) estimated that in the Lobster fishery in lobster fishing area (LFA) 34 and 41, 49% and 86% of captured Cusk were dead. This is likely an underestimate of PCM because discarded Cusk do not survive given the propensity for the stomach to evert during gear retrieval (Harris and Hanke 2010). It is assumed here that post-capture mortality is 100%.

Gavaris et al. (2010) undertook a comprehensive analysis of the 2002 – 2006 observer information for NAFODivs. 4V to 5Z. While noting that observer coverage was generally too low to provide reliable estimates of discard rates, they give an indication of which fisheries are most likely discarding Cusk, these being longline, trawl and lobster trap. Discard rates in longline and trawl fisheries operating in NAFODivs 4VW – 5Z were generally small (Table 19), averaging less than one percent. Thus, discards were not computed for these fisheries. However, the discard rates in the lobster trap fishery were 2 – 5% and this could be significant given the large amount of lobster gear fishing off Southwest Nova Scotia.

Two subsequent studies have been undertaken focused on the lobster fishery operating in Southwest Nova Scotia. The first was conducted during 2005 – 07 with sampling focused in LFA 34 and some undertaken in LFA 41. The design and results of this study are available in Harris and Hanke (2010).

The second study was conducted during 2009 – 10 with sampling effort again focused on the lobster fishery off Southwest Nova Scotia. The results of this study were peer reviewed at a DFO Science meeting in August 2010 but have not yet been published. The findings reported here are preliminary and were provided by Pezzack (2011), who noted that no samples were available for LFA 35 – 38 and no Cusk were observed in LFA27 – 32. A comparison of the 2005-06 and 2009-10 studies is provided in *Table 20 and a comparison of the bycatch rates in the lobster fishery as determined by Gavaris et al. (2010), the 2005-07 and 2009-10 studies are provided in *Table 21. For LFAs 33 – 34, in the order of 200 – 400 t of Cusk were estimated to have been discarded while for LFA 41, the discards were estimated in the order of 22 – 25 t. Pezzack (2011) estimated that almost 3 Cusk (13 kg) are discarded for every ton of lobster caught in LFA 33 and 34 respectively. Discards are higher in LFA 41, being about 37 kg of Cusk discarded for every ton of lobster caught. These are very low rates of discards.

^**Please note: Table 20 and Table 21 do not appear in this report as they were still considered DRAFT on publishing date. Please contact the Secretariat at: COSEWIC E-mail for a copy of final tables.

^[*]Table 20. Comparison of 2005-06 and 2009-10 Cusk bycatch studies (from Pezzack 2011).

^[*]Table 21. Comparison of Cusk discards in Southwest Nova Scotia Lobster fishery determined by Gavaris et al. (2010), the 2005-07 study of Harris and Hanke (2010) and the 2009-10 study of Pezzack (from Pezzack 2011).

^*Please note: Table 20 and Table 21 do not appear in this report as they were still considered DRAFT on publishing date. Please contact the Secretariat at: COSEWIC E-mail for a copy of final tables.

Lobster landings in LFAs 33, 34 and 41 since the 1999-2000 fishing season have been in the order of 20,000 t annually. The discard rates noted above imply that Cusk discards in the order of 250 – 300 t per year have been occurring since 1999-2000 (Table 22). This compares to average Canadian reported landings of 500 – 1500 t during this period (Table 16). While Cusk discard rates in the lobster fishery are very low and also highly uncertain, discarding in the Lobster fishery is likely a significant proportion of the recent total catch of Cusk.