Trout in Warming Rivers

61°F/16°C is the new 68°F/20°C

We were right up in the hills in one of New Zealand’s premier South Island backcountry rivers. This river gets a lot of guide pressure as it’s in close proximity to a well-known fishing lodge.

I’d fished it a few times prior to Christmas in cooler conditions and had a ball so it was top of the list to share with an out-of-town friend. Big browns rising to dries in deep pools, tinged glacial blue. We drove up to a section I’d been priming my visitor for. “Look out for a couple of large fish weaving ahead of those rocks. Let’s tie on a large stonefly and be ready.”

Footprints on the bank greeted us instead of feeding fish. The river looked low and barren. Instead of the gasp that immediately follows plunging my hand into the icy-cold stream, it was pleasantly temperate. Unusually warm, to my touch at least. We had enjoyed a month of excellent weather and this had clearly impacted the headwaters.

Walking a few hundred metres, we started to spot fish – not actively feeding, but almost cowering at the edges of the runs, seemingly seeking more highly oxygenated water, but very lethargic. One could almost wade out and touch the fish, they were so docile. Fishing was called off for the day and we headed towards a local lake instead.

Let’s not put our heads in the sand. Low river flows, warming water temperatures combined with greater summertime angling pressure is not good for trout.

The latest analysis by researcher Jamie Madden for KeepFishWet, released on July 1 this year has complied evidence that suggests the optimal cut off point for angling should be below the previously acknowledged benchmark of 68°F/20°C.

As stated in the report, “This is a great example of how we use science to derive best practices for catch-and-release fishing.”

It’s practical to simply quote the takeaways from this research (all bullet points are credited to this report) for us all to consider:

• Although there are slight differences among thermal tolerances of steelhead, rainbow, brook, and cutthroat trout, the angling thresholds (the temperature that indicates the beginning of an exponential rise in mortality) are lower than the 68°F/20°C commonly shared.

• I found that the angling threshold is around 61°F /16°C for steelhead, rainbow, brook, and cutthroat trout. This number was determined from a combined 24 mortality studies and 9 sublethal impact studies and keeps temperature-based mortality under 5-10%.

• Brown trout are known to withstand slightly warmer temperatures. With the data available (4 mortality studies and 1 sublethal effects study), I suggest an angling threshold of 66°F/ 19°C*.

• Comparatively, bull trout occupy a significantly colder niche than the other species examined. Combining the mortality results from one 2020 study with sublethal data and feeding temperature ranges, I suggest an angling threshold of 54°F/12°C* for bull trout.

  * This is our best guess from the limited current literature combined with known life history information of this species. More research is needed to confirm or adjust this number.

Chilling evidence from a rollup of multiple studies. The findings are worth casting your eye over. It certainly gave me pause for thought. Is it really worth chasing fish in water over 61°F/16°C?

A good start is to be informed about the temperature of the water we’re casting into. This can help make an informed decision and perhaps provide an answer to any unusual fish behaviour. Like all moral judgements when fishing on your own, you’ll need to make your own decision about whether you believe fighting a fish during C&R angling will unnecessarily affect their mortality.

For US$30, Fishpond sells an excellent Riverkeeper digital thermometer. I’m sure you can source one for only a few bucks on the big online marketplaces. Let’s make this valuable fishing tool our number one 2024 summer gear investment.

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