Most of us have made peace with catch and release. You wet your hands, keep it short, hold it upright until it kicks off, and you assume the fish swims away and gets on with its life.
Usually it does. In one specific set of conditions it very often does not, and the number is bad enough to be worth rearranging your summer around.
The ladder
A 1990 Maryland study of striped bass caught on artificial lures, in water above 25 °C — 77 °F — measured release mortality against salinity (RMC 1990, as reported by the Maryland Department of Natural Resources):
| Salinity at release | Died |
|---|---|
| 7.7–8.3 ppt | 1.8% |
| 0.7–4.2 ppt | 38.8% |
| 0.0 ppt | 70.4% |
Read the bottom row again. In warm fresh water, seven released fish in ten did not survive.
And the top row is the same species, the same season, the same gear. Mildly brackish water — the kind of salinity you get a little way down almost any tidal river — brought it to under two percent.
That is a fortyfold difference produced by nothing you did with your hands.
Why salinity of all things
The mechanism was worked out by Cech and colleagues in 1996, and it is about what happens after the fight rather than during it.
A hard fight leaves a fish with disrupted osmoregulation and electrolyte balance — its internal chemistry has been thrown out, and it has to get it back. In brackish water the surrounding salinity buffers that recovery. In fresh water it does not, and the fish is trying to repair a chemistry problem in the medium least helpful for repairing it. Add the metabolic cost of warm water and the margin disappears.
In the Maryland studies that held fish in pens afterwards, the ones that died did so after landing, not at the boat — most within six hours, nearly all within a day (Lukacovic). A fish that swims off strongly can still die later. You will not see it, and you will not know.
Air temperature stacks on top
A separate Maryland study, of bait-caught fish held in pens for three days in 1999, looked at the air rather than the water.
Shallow-hooked fish died at 0.8% when the air stayed below 95 °F. On July days between 95 and 105 °F, that rose to 17.2% (Lukacovic, Maryland DNR).
Same hook placement — a twentyfold change driven by how hot it was on the water.
What the season does
Massachusetts's own release study, run by the Division of Marine Fisheries in a saltwater pond in Salem, put overall mortality at about 9% (Diodati and Richards 1996). The season moves that number. A 2024 tagging study on Maryland's Patuxent River, lure-caught fish again, put release mortality at about 11% in summer, 7% in spring and under 1% in fall — small samples and wide error bars, but the same direction as everything above (Nelson, reported in Kennedy et al. 2025).
So the average across a year hides most of what matters. A carefully released October fish is very likely to live. A July fish released at the top of a river is a different proposition entirely.
What to do with it
Know the salinity where you are standing. This is the lever, and most of us have never thought about it. The upper reaches of tidal rivers, herring runs, and the fresh side of a dam are where the 70% number lives. If you are fishing there in July and August, that is the thing to change.
If you fish fresh water in the heat, fish the brackish side. The ladder is steep at the bottom, and a little salt at release is worth a great deal.
Watch the air, not just the water. On a 95°F-plus day, keep it in the water, unhook it in the water, and skip the photo. That is where the twentyfold sits.
And consider not fighting the fish at all. In the hottest part of summer, at the freshest end of the system, the honest arithmetic says a fish you hook is a fish that probably dies. If you would not keep it, the kindest version of the sport that week might be to go somewhere saltier.
How StripedLines uses this
StripedLines does not show a release read today. The numbers above are the tool: warm, fresh and hot out is the combination that kills fish, and removing any one of the three helps.