36% of the fish had died within the first week of release. Another 8% had tags that released early, but the team were unable to determine whether they were dead when this happened. Twenty fish survived the tagging and release. Even though there was no clear evidence as to the cause of mortality, size seemed to be an important factor to fish survival. Most fish larger than 85cm died whereas smaller fish survived at a high rate.
Susanne McDermott said that initial mortality may have also been due to trauma from being brought up from deeper depths.
‘Barotrauma may have occurred as the fish’s swim bladder expands when it comes to the surface,’ she said.
‘Others were eaten by predators or caught by fishing boats. The data offers clear information on where the fish were when the tags popped up, while depth and temperature data are helping us recreate the fish’s travel history.’
The team found that seven out of nine fish went to Seguam Pass, a major feeding ground during non-spawning season between the Bering Sea and North Pacific Ocean. One went further west to Petrel Bank and one went east to Umnak Island, both about 400km from the release site.
Pacific cod also migrate more than 300nm from spawning areas to summer feeding areas, with the migration to feeding areas happening within weeks after spawning. The tags also showed a great deal of vertical movement as well as distance travelled. On average, the fish rose and descended 200 feet daily, and fed in deeper, highly productive areas where a lot of fish congregate.
‘Some fish didn’t show any difference in movement between day and night but others did,” she said.
‘At night they were deeper, and shallower during the day, which suggests that they feed on something in the water column in the daytime. We weren’t aware that cod moved that high up in the water column to feed. The fish also knew exactly where they were going – most fish went straight to Seguam pass, a known place of high productivity where there’s lots of food. They weren’t waiting around. They went right away, and once there, they stayed. That was a really exciting, new thing to see.’
As the study continues, with more tags being released and more fish being tracked, the team hope to have an even better understanding of migratory pathways and whether the fish spend time in areas that surveys can’t reach. In summer 2019, a similar project was conducted in the Bering Sea in which 38 fish were tagged and released. With data from 16 tags already recovered and the remaining 22 scheduled to appear soon, the team will compare them with the Aleutian Islands project to build a bigger picture of Pacific cod movement, with the aim of increasing study area as well.
The Aleutian Island project has been important to the local fishing community and is a significant example of fishermen’s expertise being used in scientific research.
‘We have a very supportive fishing industry that’s interested in research,’ Susanne McDermott said.
‘It’s been a very good chance to collaborate. The fishermen know plenty about spawning locations and timing and that really helped us decide where to go and when. They also knew a lot about where to catch the fish that we wanted, which is essential if you want to hone in on a species in particular life stages or events.’
‘It’s been a great opportunity to get information from local fishers while bringing in our scientific approach. It was extremely collaborative right from the start,’ she concluded.