Blue shark (Atlantic and Pacific populations) COSEWIC assessment and status report: chapter 9

Fisheries (Bycatch)

Atlantic

Catch Mortality of Blue Sharks in Atlantic Waters

Campana et al. (2004) estimated the catch mortality of blue sharks in Canadian waters to be on average 1000 t per year since 1986 based on the combination of reported landings, observed bycatch, estimated non-observed bycatch, and estimated hooking mortality.

Reported landings includes landing data from recreational and commercial fisheries (including foreign vessels) but since most blue sharks are caught as bycatch and discarded, reported landings do not provide much insight into the actual catch and mortality. Observed bycatch is the amount of blue shark caught (landings and discards) reported by the International Observer Program (IOP) which has 100% observer coverage on foreign vessels and ~5% coverage for Canadian vessels. To account for the non-observed Canadian fishery, the encounter rate of blue sharks observed in the foreign fishery was extrapolated to Canadian vessels to estimate the minimum non-observed bycatch. The proportion of blue sharks comprising the non-observed bycatch varies depending on season and fishery type and therefore a unique bycatch proportion was attributed by season and fishery. Furthermore, known under-reporting of blue sharks within the observer program was accounted for by using only datasets with at least one blue shark resulting in a maximum non-observed bycatch proportion. The average of the minimum and maximum proportions was used to calculate the overall non-observed bycatch.

Virtually all the catch of blue shark is bycatch and therefore calculating the survival rate after release is critical for understanding the hooking mortality. Hooking mortality has been estimated based on a small study (n=105) that categorized the state of the animal upon retrieval as healthy (38%), injured (44%), or dead (18%) (Campana et al. 2004). It was arbitrarily assumed that half of the injured animals died and therefore the overall hooking mortality was assumed to be 40%. Note there was no accounting for “drop off” mortality (animals that were caught but fell off the hook before being brought on board). Figure 10 shows the overall mortality in Canadian waters, which on average is approximately 1000 t per year (1986-2003). Annual catch mortality from 1996 to 2003 has been relatively stable averaging less than 700 t per year. The decline in catch mortality from 1996 onwards has resulted from a decrease in foreign fishing combined with a decrease in Canadian catch for swordfish (Campana et al. 2004).


Figure 10: Total Catch by Source for Blue Sharks Caught in Atlantic Canadian Waters

Figure 10: Total catch by source for blue sharks caught in Atlantic Canadian waters.

From Campana et al. 2004).


Campana et al. (2004) extended the analysis described above to account for catch mortality in the entire North Atlantic. Based on international data from 2000, they conservatively estimated that more than 100 000 t of blue shark were caught which resulted in an approximate catch mortality of 37 000 t in that year.

An alternative and totally independent technique from the one described previously was also applied to calculate North Atlantic mortality. Using exploitation rates from tagging studies and fishing mortality rates from catch curve analysis, overall North Atlantic catch mortality was estimated to be 26 000 t/yr (Campana et al. 2004).

Given the widespread and migratory nature of this species, it is clear that mortality due to fishing in Canadian waters is a small fraction of the overall mortality. Furthermore, as inferred from the age and maturity of animals in the Canadian catch, it is very unlikely that the Canadian fishery has a disproportionate impact on a critical population component such as mature females.

Exploitation Rates from Tagging-Atlantic

Tagging studies (described above in the Dispersal/Migration section) have been used to estimate exploitation rates in Canadian waters (Campana et al. 2004). In the Canadian study (1961-1980), annual exploitation rates were always less than 1%. Similarly, the NMFS tagging study resulted in a mean annual exploitation rate in Canadian waters of 0.78% for the years 1992-2002. Most (93%) of the Canadian tags were applied before 1972. As a crude comparison between the two tagging periods (1961-1972 and 1992-2002), Campana et al. (2004) compared overall tag return ratios and found that exploitation rates increased from a mean of 0.009 to 0.089 (~tenfold) under the assumption of similar reporting rates. This increase in apparent exploitation was similar in magnitude to the approximate tenfold increase in North Atlantic longline effort (Figure 11; ICCAT 2005).


Figure 11: Trend in Effort for the North and South Atlantic Longline Fleet (1956-1997)

Figure 11: Trend in effort for the North and South Atlantic longline fleet (1956-1997)

Figure from ICCAT 2005. North and South refer to hemispheres.


Campana et al. (2004) also compared the proportion of tags recaptured in Canada with those caught elsewhere in the Atlantic. This comparison found that about 1/3 of the total fishing mortality occurred in Canada, but this is likely an overestimate due to better tag reporting occurring in Canadian waters than in international waters.

Overall, it appears that the annual exploitation rate in Canadian waters is probably less than 1% per year.


Pacific

Canadian Fishery Data-Pacific

Since 1996 there has been 100% observer coverage in the “Option A” groundfish trawl fishery, which accounts for the majority of the groundfish landings on British Columbia’s coast. From 1996-2004 there have been a total of 5737 kg of blue sharks caught by the trawl fleet resulting in a mean of 637 kg/year. The catch is concentrated along the west coast of Vancouver Island and off the south end of the Queen Charlotte Islands (Figure 12A). Actual mortality rate (i.e., survival after discarding) of trawl caught sharks is unknown.


Figure 12: Distribution of Blue Shark Catches Between 1996-2004 in Commercial (a) Groundfish Trawl and (b) Hook and Line Fisheries

Figure 12: Distribution of blue shark catches between 1996-2004 in commercial groundfish trawl and hook and line fisheries.

Source: DFO PacHarvHL and PacHarvTrawl databases.


The hook and line fleets (i.e., halibut, sablefish, dogfish, lingcod, and rockfish) are not permitted to fish for or retain blue sharks and therefore all catch records are from either logbook data filled out by fishers or in recent years from the observer program. The observer program began in 2001 with a maximum coverage of 15% in the hook and line fisheries. Voluntary reporting of bycatch species in logbooks is of limited use for understanding the actual catch. The implementation of the observer program, even with limited coverage, has provided a means to estimate actual catch. From 2001 to 2003 there has been a sharp increase in blue shark records (Figure 13). Prior to 2001, reported blue shark annual catch ranged from 0-0.84 t. From 2001-2003 reported catches range from 3.8-7.7 t which is conservatively a five to tenfold increase from pre-observer years. Given that observed trips account for only ~10-15% of the overall fishing effort, annual catches are likely conservatively five times greater than presently reported or about 20-40 t per year. There is no basis for estimating mortality. The sablefish fishery catch database has only two records of blue sharks since 1996 indicating that there are likely few encounters with blue sharks in this fishery (DFO PacHarSable database).


Figure 13: Reported Catch of Blue Shark by British Columbia’s Hook and Line Fleet Between 1997-2004

Figure 13: Reported catch of blue shark by British Columbia’s hook and line fleet between 1997-2004

Note that the increase after 2000 is due to the implementation of a partial observer program covering approximately 10-15% of the trips whereas prior to 2001 data was voluntarily recorded in logbooks. Source: DFO PacHarvHL Database.


Blue sharks are also reported caught in small numbers by the salmon troll fleet. Since 2001, some segments of the salmon troll fishery have operated year-round, but blue shark encounters, as reported in logbooks, are confined to the months of July to early October. From 2001-2004, a total of 42 blue sharks were reported as caught and released in the salmon troll fishery. Furthermore the recreational fisheries for salmon and groundfish likely encounter blue shark but catches of sharks in those fisheries are unknown.

Estimated mortality from fishing in Canadian waters is small in comparison to estimated catch throughout the North Pacific. Bonfil (1994) estimated annual blue shark catch in the North Pacific is approximately 2 million individuals or 39 000 metric tons. Information pertaining to blue shark distribution and migration supports the notion that sharks caught in Canada are part of a much larger North Pacific hemispheric population. Canada’s Pacific waters are part of this species’ normal range but current domestic fisheries are seemingly having a negligible impact on the population and account for perhaps 0.1% of the population’s mortality (assuming 100% discard mortality). Furthermore, limited evidence suggests that animals encountered in Pacific Canadian waters are primarily immature individuals.

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