Submitted by Anonymous on Fri, 07/25/2008 - 16:20
Forums
Kuril Islands Biocomplexity 08

I know you've moved on to the off-shore trips, but I'm curious about the fish counts.  The first counts reported suggested that you were mostly finding the mackerel, but that it was 60:16 with other species. So, then on the next report, there were nearly 200 mackerel and 0 other species?  Were the crew fishing in a different fashion?  Were they throwing back the other species before they were counted?  Were you in a different location?  Just curious and wondering too how many counts like this you'd need to feel like it was a good representation from which to try to generalize to archival records?

Hope you're having fun.

Jenny E

Mon, 07/28/2008 - 16:59

KBP Team

Leave it to my older sister to double-check the statistical soundness of my science! Thanks Dr. E (sis)!
There are a couple of things that could be going on to explain the striking differences in species composition between the two localities (separated by 10-15 km, but within sight of each other). We can rule out changes in gear or selectivity-they were fishing the same way, and they really don't ever throw anything back [what doesn't get used for meals in the galley will be frozen or dried to take to their homes after the cruise-see photo].

 
Locality and habitat might explain some of it-the Atka mackerel are more generalists than the other species [the others prefer rocky substrate, with some kelp for cover]. The other thing that might be going on is that the mackerel are a schooling fish, and can be quite aggressive predators, perhaps to the exclusion of other species in the area. On several occasions, people were catching multiple fish on a single line (with 2-5 hooks), and we would even see fish following the line to the surface from 10-12 m deep! Now, that still doesn't answer the question of whether or not the other species were there and just not getting access to the baited hooks.
As for the question of how many repeated samples I would need to have any confidence in my numbers? That's always a tough one, right? It is especially tough in fisheries science because modal tendencies in fish populations can be really strongly influenced by a particularly strong year-class. That is to say, if 2005 were a really really good year for Atka mackerel, that cohort will influence the length-frequency distribution for several years.
The main thing I have going for me with the Atka mackerel is that it is a very important commercial species, and is monitored quite closely by full-time fisheries scientists. So if I were ever in a position to actually use these data, I would bolster my data with other, more extensive data sets.
And for comparison with the ancient materials? Unless there have been really drastic changes, it isn't very likely that we would see any differences. But there are some really fascinating case studies out there [I am thinking, in particular, about drastic reductions in Atlantic cod (Gadus morhua) size in the western North Atlantic from prehistoric to modern times] that make it least worth looking into.
Of course, all that pre-supposes that we'll have an ancient sample to work with, which we don't yet this summer (although we do have a large sample of fish, mostly in the Greenling family, from a site we worked on in 2006 and 2007).
--(the other) Dr. E.