Posted on July 17 2012
We were thrilled when Brian Morrison – Alaska West guest and fisheries biologist – volunteered to put together a series of articles for us about the science behind some of the fish we’re so lucky to chase in Alaska. We think Brian’s insights are really interesting and hope you will too.
Western Alaska Trout Lifecycles
As a biologist working with naturalized salmon (Chinook, Coho, and Pink) and steelhead on the Ontario side of the Great Lakes for the better part of 15 years, I’ve always been intrigued with these species within their native range. I had always dreamt of viewing them in intact ecosystems, expressing their original suite of life-history and behavioral characteristics, unaltered through heavy exploitation or habitat degradation. I was fortunate enough to experience this on the Kanektok and Arolik Rivers in 2009 and 2011, where I had a snapshot of these ecosystems and fish. This will be the first of a series on the life-history of some of the more popular species targeted by anglers on these two rivers. This article will focus on Rainbow Trout.
Prior to my trips, I looked around to see what, if anything, was in the literature describing the population dynamics of Rainbow Trout on either of these rivers. I managed to find a few reports through the US Fish & Wildlife Service and the Alaska Department of Fish & Wildlife from the 1990’s. Based on these, the age of fish sampled on the Arolik River ranged from 3 to 11 years, with fish aged 6 or 7 years being the dominant age groups sampled. Age 11 seems to be the oldest record for any population of Rainbow Trout in any Kuskokwim Bay (western Alaska) population, or most anywhere else in the world. The length of fish (fork length) ranged from 250 mm (just under 10″) for a 3 year old to 700mm (27 1/2″) for fish at age 11.
Figure 1. Average Length (mm) at age for Arolik River Rainbow Trout (AKDFW 1991-93; USFWS 1996).
Based on the information that I could find, I was curious of the age of some of the larger Rainbow Trout, as there seemed to be a gap in the knowledge for the oldest fish in the population, based on a small sample size of fish older than age 8 being analyzed. Results from scale sample analysis from a couple of Rainbow Trout caught from the Arolik River in 2011 show the following:
One was a male, 651mm fork length that was interpreted to be 10 yrs old, and the other was a female that was 513mm fork length, which was interpreted as an 8 yr old. Both fish showed growth patterns similar to a steelhead, in that they spend the first 2-3 years in habitats that allowed for very slow growth (This is likely off-channel habitat off of the main river). Once they enter the main-stem of the river and larger braids, they start to put on some serious growth until they are sexually mature, at which point growth begins to slow down (this is similar to steelhead, which do most of their growing in the ocean vs. the stream – in this case the larger river is the ‘ocean’ and the off channel habitats are the ‘stream’).
Based on past studies, Rainbow Trout in the Arolik River were found to be larger in length at age and have a greater maximum age, and maximum length than other western Alaska populations. This may be confounded because Rainbow Trout that utilize the lower parts of rivers and estuaries (perhaps even have forays into the near-shore part of the Bering Sea?), such as on the Kanektok River, have not been studied and tend to be the largest fish in the population, where fish up to 840mm (33″!) have been caught. Anglers have also noted that the coloration of these fish are different from fish caught in upstream areas, with the fish lower on the river still displaying leopard spotting, but are much more chrome than upriver fish.