Article by Capt. Mark Sampson
Looking at my seawater temperature gauge I saw that it was reading 81.1 degrees. "Whoa!" I thought. "That seems awfully warm for this time of the season". Sure, it was late morning on a hot, calm day and we were only about 5-miles offshore, but temps in the 80's are more August stuff and we were only a week into July!
So to confirm my thoughts I pulled out my 2022 log book and saw that during the same week a year ago we were recording temperatures only around 72-74 degrees. I'm guessing that a lot of anglers would likely conclude that a temperature variation of 9 degrees would have created quite a difference in the fishing last year vs last week, and maybe they'd be right - maybe. But before we get into those differences perhaps we should consider some of what goes on above and below the waves that affect water temperatures and the fish we hope to catch.
First we have to remember that our temperature gauges most of us have on our boats pretty much only read the surface layer of the water, which can be dramatically different from the temperature just a few feet down and where the fish we're trying to catch actually reside. On a hot calm day the sun will warm the surface making an even greater difference between what's up top and what's down below. On a choppy day the surface water will mix a bit with what's underneath it and our readings will be a little more true to form for the upper levels of the water column. But unless they’re a diver, and swim down or have a temperature probe they can drop many feet below the surface, few know what it’s really like at any given time 10 or more feet below their boat. I’ve done both and can attest that there can be a dramatic difference in the temperature our boats are floating in and that where we have our baited hooks. But that’s not to suggest that surface temperatures aren’t important in directing us to where the fish might be, they certainly are. It’s just that we have to keep what they’re telling us in proper perspective and use that “best available science” as a guide where the fish might or might not be, not a rule.
Water temperatures are affected by many factors including west wind turnover, Gulf Stream, wind speed and direction, upwellings (from shoals), rain (inshore), currents, tides and of course, the air temperatures.
West wind turnover is when a prolonged wind from a westerly direction pushes the ocean’s warm surface layer to the east and away from the beach. As the warm water moves offshore it is replaced right at the beach (the surfline) with the cooler water that has been below it. As long as the wind remains out of the west, the cooler water continues to be pushed farther and farther offshore until the entire area off our coast is nothing but cold water with temperatures that might drop from the upper 70’s to the lower 60’s in a very short amount of time. This phenomenon is obvious not only to boaters with temperature gauges but also to anyone inclined to take a swim from the beach and find that even though it’s the hottest day of the summer the ocean suddenly feels like ice! The cool water will also cool the air over the ocean enough that boaters might find themselves donning sweatshirts in the middle of a heatwave.
So what does all this mean to the fisherman? First we have to remember that what we're seeing is not necessarily the best representation of what we have beneath our hulls, at least when it comes to warm water. The temperature our gauges tell us is always going to be a little, or sometimes a lot, higher than the water down deep because warm water rises and cold water sinks. And since most of the fish anglers are typically trying to catch might spend more time down below the warm surface layer than right at it, it can be tough sometimes to accurately conclude that you’re fishing in the best temperature of water for a certain species even if you know what temperature range that type of fish prefers. However, since sea surface temperature gauges are typically all we’ve got to go by, they’re still the best tool we have to help figure out if we’re at least fishing close to the right ballpark.
Something else to consider about water temperatures and how they affect the fact that even if temps are just right for XYZ fish, they might be way out of line for whatever those fish want to feed on. If the little critters leave or avoid the area because it’s too hot or cold then the larger predators might just move out with them. I don’t know if it’s a water temperature thing or not but did you notice that the last couple years we haven’t had the huge schools of bunker (menhaden) off our coast? Apparently they’ve been showing way up north in places they’ve never been before and it hasn’t exactly been a banner couple of years for the cobia, sharks, stripers, bluefish and so many other fish known to feed on the important bait fish. Have they traveled north following the bunker? Is all the temperature related?
One great advantage to offshore anglers is the ability to use their gauges to see temperature differences or "breaks" when trying to catch pelagic species such as tuna and billfish as these fish will often either hang out along the break or just stay primarily on the warm or cold side of it. While these breaks can be 5-10 degrees different, 2-3 degrees is more common and enough to show that the water is, for some reason, different. Maybe it's the west edge of the Gulf Stream, or an eddy that spun off of it. Maybe the difference is from some current that pushed into the area, or possibly from an upwelling of water produced when a current flows across an underwater hump, or the edge of the Continental Shelf, or maybe it’s from water flowing out of an inlet. Whatever the reason for being, temperature breaks can very often produce fish because they can effectively create a wall that predator fish or the bait they want to feed on aren't inclined to cross.
So anyway, how important is that seawater temperature gauge on a fishing boat? Not at all, maybe a little, possibly a lot. It all depends upon where, when, and what you’re pursuing. Overall they’re a great tool for any fishermen to have, and since just about every depth finder these days has a temperature gauge built into it, there’s almost no excuse not to have one on your boat. The coastal waters are a dynamic and complicated ecosystem, and knowing the surface seawater temperature in any given place won’t always tell you where you will or will not find fish, but add that information to whatever else you might know about what’s going on under and around your hull, and you might just be able to figure it out.
Getting back to a couple weeks ago when we encountered 81 degree water compared to the 74 we had exactly one year earlier - my log showed the fishing activity was almost identical. Go figure! §
