Tope (Galeorhinus galeus) COSEWIC assessment and status report: chapter 8

Population Sizes and Trends

Search effort

Tope was first recorded from British Columbia waters in 1891 by Ashdown Green who reported it to be rather common along the coast (Clemens and Wilby 1946). At present time there are no indices available to assess tope population trends anywhere in their northeast Pacific range. The National Marine Fisheries Service triennial bottom trawl survey (1977-2001) has only 23 tope captures between California and waters off southern Vancouver Island over eight survey years. Only two records are from tows north of 48º (NMFS Triennial survey, unpublished data). The International Pacific Halibut Commission annual set line survey has 45 records of tope dating back to 1996 but they are not caught regularly enough to develop an index of abundance (Appendix 3). There have been no tope captured by Canadian research surveys (GFBio database).

Abundance and trends

There are no indices of tope abundance anywhere in their northeastern Pacific range. The only research published on tope in British Columbia is from Barraclough’s (1948) account of the fishery and a brief mention by Westrheim (1950). Barraclough estimated that 40% of all tope livers landed in British Columbia were landed by sunken gillnet boats fishing primarily for spiny dogfish from May to October in northwestern Hecate Strait (see Figure 6). We examined catch and effort of present-day fishing in this area between 1996 and October 2005 and found no records of tope catch over the same months despite 7243 hours of trawling effort and 1632 sets made with hook and line gear (Figure 9). It should be noted that the gear types presently being used are quite different than the preferred historical gear type involving sunken gillnets.

Figure 9. Overlay of fishing effort by trawl vessels (red circles) and hook and line vessels (grey circles) fishing between 1996 and October 2005 on historical tope fishing grounds. Source: PacHarvHL and Trawl databases.

Figure 9.  Overlay of fishing effort by trawl vessels (red circles) and hook and line vessels (grey circles) fishing between 1996 and October 2005 on historical tope fishing grounds.

The absence of present-day tope catch records from this region is difficult to interpret. On one hand there is considerable present-day fishing effort and therefore one would expect that if present, the sharks would be occasionally captured by either commercial or recreational fisheries. Worldwide, tope are regularly captured by trawl gear as bycatch and in some areas are actually targeted by this gear type so it would be expected that if present in northwest Hecate Strait they would appear in observer catch records (Walker 1999).  Likewise, bottom-set longlining is another common technique for capturing tope in many places in the world and was also used in British Columbia during the historic directed fishery. Based on the fishing effort it appears that tope have not been present in this area in recent years (1996-2005).

On the other hand, the world’s foremost expert on tope fisheries reports that this species is not an easy quarry (Walker 1999). Fishers need considerable experience and skill to successfully find and catch them because of their schooling and highly migratory characteristics. If fishers are not directly targeting tope they may simply not catch them. Longline effort, although quite extensive, is primarily targeting lingcod, spiny dogfish and rockfish which may not overlap with the preferred habitat of tope. Another factor is that only a small percentage of the hook and line fleet is monitored by at-sea observers and therefore the actual present day catch of tope in this region is not fully reported. Finally, Hecate Strait represents the northern extent of tope distribution and therefore environmental conditions may play a role in their inter-annual distribution. It is possible that the abundance of tope in Hecate Strait during the 1940s was due to suitable environmental conditions.

In conclusion, the absence of present day catch records in an area of known historical abundance is of interest but there is presently insufficient information to fully explain this observation. Overall abundance and population trends in Canada’s Pacific waters are unknown.

Rescue effect

British Columbia represents the northward extension of tope range in the northeast Pacific. Rescue effect from southerly waters is likely possible but is presently unknown. The extent of a rescue effect depends on both the rate of interchange between U.S. and Canadian waters as well as the current abundance. Populations in U.S. waters have not been studied in over 50 years (Ebert 2001). Based on a very limited tagging study in the 1940s there is indication that some portion of the otherwise more southerly centred population migrates and utilizes Canadian waters. It is suspected that tope are primarily seasonal visitors to Canadian waters; however, trawl observer data indicates that they can be caught year round except for March and April (Appendix 2). In other areas of the world, tope are known to make large latitudinal seasonal migrations (see review by Walker 1999) suggesting the same may occur in the northeast Pacific. Overall, it is reasonable to assume that if populations in U.S. waters are healthy they are potentially abundant in Canadian waters providing the environmental conditions are suitable. Similarly, if the tope population in U.S. waters is depleted it would be expected that abundance in Canadian waters would also be diminished.

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