Decrease in Prawn Abundance of Desolation Sound

**Observation by Conlan Magel:

Over the last 20 years, I have fished the British Columbian coastline from Howe Sound to Desolation Sound. Specifically, in Desolation Sound I would lay approximately 25-30 prawn traps within a 2 week period during August. The month of August follows British Columbia’s commercial prawn fisheries closure and is known to have less prawn production. Ten years ago a trap would produce on average 10 -12 prawns with only 5 or so traps being empty. However, over the last 10 years, the number has been decreasing. This year 32 laid traps produced a total of 55 prawns, with 27 traps coming up empty. It is apparent that this area of coastline has experienced a sharp decline in its prawn populations. This may be due to increased fishing pressure from commercial prawn fishermen.

While in the Desolation Sound region I took the opportunity to ask other local fishermen about their prawning success. A local of Cortez Island informed me that each passing year it has become harder to trap prawns in the area. Furthermore, an individual who owns a fishing lodge in Desolation Sound also supported the observation of the decline of prawn populations in the area and claimed that commercial prawners have been fishing the area heavily over the last few years.

In British Columbia, the management of the pacific prawn fisheries falls on the federal department of fisheries and oceans. Laurie Convey, a resource biologist with the department of fisheries and oceans could provide better insight and understanding on why the prawn population of Desolation Sound is declining. Laurie Convey is DFO’s pacific fisheries specialist in prawn by trap management and can explain how prawn populations for Desolation Sound are monitored annually.

Tom Okey writes:

I went to the Global Fishing Watch Map and zoomed in on Desolation Sound, and based on the AIS (Automatic Identification System) data, which commercial vessels are supposed to use, it does not appear to me that there has been excessive fishing pressure for prawns in Desolation Sound over the last decade (when these data have been available). I really like the personal 'Catch Per Unit Effort' (CPUE) example that Conlan Magel provided, as supported by two additional prawn trappers -- it is evidence of a decline in prawn abundance / biomass. However, given the lack of evidence of increasing commercial prawn trawler activity in Desolation Sound, I think it would be worth it to consider alternate explanations for this observed decline. On the other hand, if the regionally broader metapopulation of this prawn is overfished (i.e., in the northern Strait of Georgia), then overfishing could be expressed as fewer prawns in Desolation Sound, especially given that prawns must aggregate (Prince et al. 2008), which is expressed as population declines at the outer margins of a metapopulation. The Global Fishing Watch / AIS data would be more supportive of that explanation of prawn declines in Desolation Sound. Another explanation for prawn declines in the area could be decreases in dissolved oxygen on the sea floor due to the effects of organic enrichment of the marine environment by finfish aquaculture activities. The Discovery Islands fish farms are located not far to the northwest of Desolation Sound. Recent marine heat waves and changes in precipitation would be other hypotheses to examine.

Literature cited Prince, J.D., Loneragan, N.R. and Okey, T.A., 2008. Contraction of the banana prawn (Penaeus merguiensis) fishery of Albatross Bay in the Gulf of Carpentaria, Australia. Marine and Freshwater Research, 59(5), pp.383-390.