Brook Trout Study Shows High Post-Spawn Mortality

Brook trout | Photo by Eric Engbretson, U.S. Fish and Wildlife Service, via Wikimedia Commons

The following article was provided by the Maine Dept. of Inland Fisheries.

Back in 1958, fisheries biologist Roger AuClair conducted a tagging study on wild brook trout originating in Moosehead Lake. Roger constructed a wooden fish weir on Socatean Stream and captured the fish during their spawning migration. This study was one of the first of its kind in Maine, and found that 65% of the male brook trout in his study died soon after spawning.

We duplicated Roger’s work in 2009 on Socatean Stream.  There is still a good run of wild trout in the stream. We were able to use more modern equipment such as radio telemetry and passive integrated transponder (PIT) tags for our study, but the results were remarkably similar. We estimated 57%-60% of the male brook trout perished as a result of spawning stress in our recent study.

We packed up our toys and moved over to the Roach River (near Moosehead Lake) in 2010 and 2011 to determine if brook trout mortality rates would be different on a larger river. In fact, they were quite comparable: 51%-57% on the Roach River. This also correlated well with a study we conducted on Chamberlain Lake in 2006, which found that 51% of mature male brook trout did not survive the spawning run. The data are pretty consistent. We can expect 50%-65% of wild mature male brook trout to succumb in the fall during after spawning.

Our radio telemetry studies enabled us to locate the dead or dying fish and determine the cause of death. Most of the tags we recovered were in places that indicated predators play a major role in their demise. Otters, mink, great blue herons, eagles, and ospreys take their toll as the tired and stressed brook trout can no longer elude them. In Socatean Stream, some of the trout actually entered the stream in August but didn’t spawn until October. That’s a long time to play hide and seek with foraging fur-bearers and avian assassins.

The actual act of spawning is rigorous as well. It’s not all flirting and buying chocolates. Male brook trout are constantly chasing and fighting other male trout during the courting process. There is non-stop action for weeks, but successful spawning is essential to maintain the population.

It’s a grim picture but it is worth painting. We are frequently asked about extending the fishing season on wild brook trout (and salmon) in the Moosehead Lake Region into October and November while they are spawning. Think of the additional stress of hooking and releasing wild fish during the time of year when they are the most vulnerable. What happens when an angler hooks and plays a female fish that is ready to deposit her egg? I know I have handled many gravid fish in my career and even when they are anesthetized, many drop their eggs and milt. And what about freshly deposited eggs in the gravel as anglers wade up and down the streams and rivers in pursuit of the catch? These issues must be carefully considered for the long-term protection of the resource.

Maine is the last place on the eastern seaboard with significant numbers of intact wild and native brook trout populations. To address public interest in fall fishing, we extended the fishing season several years ago on all waters where the fishery is maintained entirely through stocking. Successful reproduction is not a concern in these waters. The hatchery truck comes every year. These are great places to provide additional fishing opportunities in the fall months.