Mortality - the word scientists give to the deaths of individuals - is not as simple as it might first look. Yes, everything that is alive is going to die, but that is not the question. Rather, how and for fisheries management, what effect mortality has on populations is the more interesting - and difficult - question. You see, not all fish are created equal. The loss of a nine inch trout is different from a sixteen inch trout or a six inch trout. I figured publishing this post about a month before the "catch and kill" opener was pretty fitting.
Population biologists categorize mortality for exploited populations - that is populations that are harvested - as natural mortality and fishing mortality. Natural mortality is dying of natural causes such as old age, disease, and predation, and catastrophic events among other natural causes. Fishing mortality gets divided into harvest mortality and handling (catch and release) mortality. Catch and release mortality, particularly when using artificial lures and flies with single hooks, is very low. That is a topic that deserves its own post but with suitable water temperatures and fish handling - minimizing their time out of the water and not letting them flop around on the bank - catch and release mortality is very low; 2-3% or less and quite often approaches 0%.
Now here is where it gets interesting - well, at least I think so. Population biology also categorizes mortality as additive and compensatory and sometimes mortality is compensatory until it becomes additive. Yeah, I've got some explaining to do. Compensatory mortality basically states that some percentage of the population will die each year so if those fish are harvested or die of natural causes, it does not matter to the population. Essentially, some percent of the population will die of natural or harvest mortality, how that happens has no effect on the total population. Additive mortality says that any mortality - natural or harvest related - reduces the total population. And to keep things interesting, there may be compensatory mortality up until a point where it becomes additive. Twenty-percent of the population may die of either natural or harvest causes, but once you reach 20.1%, mortality become additive. The rather obvious problem is knowing what the level of compensatory mortality is before it become additive. Essentially, for fisheries management, we want to be harvesting most populations below where additive mortality kicks in.
Mortality and Population Density
A few months I wrote about models and the BIDE approach to modeling. Populations grow through births and immigrants and are reduced through deaths and emigrants. All models and evaluations of populations begin here. Natural mortality tends to increase the closer populations get to carrying capacity and is reduced the further the population is from its carrying capacity. And it is important to understand that the different streams and even reaches within the stream have different carrying capacities. This generally means that some streams will withstand a significant amount of harvest (it will be compensatory) where as in other streams, harvest is all additive or is additive after a small amount of harvest.
Variation in Mortality Among Species
Mortality varies pretty greatly among species. Basic population ecology tells us that most fishes that are born are not going to survive to adulthood. Most fishes fall on the r-selected end of the spectrum - they produce a lot of eggs, few of which survive to adulthood to reproduce. Then digging deeper, there is a lot of variation within fishes. Take the centrarchids, members of the bass and sunfish family which included Bluegill and the "black basses" (Largemouth, Smallmouth, and others), which have parental care for their eggs and offspring. Their eggs are more likely to survive to adulthood than are fishes that have no parental care. Parental care is a rarity in the fish world. The tradeoff is typically that the broods of fishes with parental care are smaller than those of non-guarding species. In the end, to replace themselves, fishes really do not need a large percentage of their eggs to live to reproduce. And it really doesn't matter if 1,000,000 eggs are produced and 0.1% survive to adulthood or 2,000 eggs are produced and half make it to adulthood. In both cases, a fish has produced 1,000 offspring that have the potential to carry on the parents' genetics.
Fishing mortality also varies greatly among species and based on their biology, reproductive output, and place on the trophic pyramid. Some fishes are better able to withstand harvest than others. To use those fancy words again, compensatory mortality is higher in some fishes than others. We can harvest more Yellow Perch than we can Walleye. More Bluegill than Smallmouth Bass. Catch and release mortality also tends to vary among species and for many species is strongly correlated with water temperature. Catch and release mortality tends to be higher with deep water fishes or when fishes are caught near their thermal tolerance. For example, coolwater fishes like Northern Pike and musky caught in August will die at a higher rate than they do in June or October. Same with trout - catch and release mortality is very low as long as water temperature is below 68*F (20*C ) or so.
Of course human choices and preferences have a pretty large effect on what gets harvested and what does not. Bass (largely) get put back (thanks B.A.S.S.) and Walleye get "released to the grease" - damn them for being so tasty. In fact, maybe you have been following the news about Walleye declining in many Wisconsin lakes. Some of it is due to a decrease in the thermal-optical habitat area (basically the intersection of temperature and light levels where walleye are successful) due to climate change but harvest has helped exacerbate the effects of climate change on Walleye populations. From the University of Wisconsin and the Center for Limnology, in a news release about an ongoing Walleye study;
They used adult walleye population estimates to set regulations that ensured a maximum harvest amount of 35 percent of the adult walleye in any given lake. The average exploitation rate for walleye stocks is closer to 15 percent, so the agencies assumed these regulations were sufficiently conservative to be sustainable.
Dr. Steven Carpenter goes on to say,
“Nature has changed,” says Carpenter. “The climate now is different from what it was in the 1980s and it’s not going back. That means habitat is decreasing and, on average, walleye stocks can’t take the harvest levels they have seen.”
Please read the rest of the story as it is an interesting tale of how climate and harvest are interacting in a way that has become unsustainable. In essence, the habitat of Walleye is shrinking due to lakes being warmer and clearer. Couple this with the fact that they are very often kept, throw in an unsuccessful year or two of spawning, and you have a recipe that leads to their decline. The solution is generally not one that a lot of anglers are going to like - we need to cut back on harvest. Climate change has in essence reduced the level of compensatory mortality in many Walleye lakes and mortality has become additive leading to a decline in their populations. And despite barstool biologists' rumblings, it is NOT musky that are responsible for it but that is a post for another day!
Harvest and the Trout Angler
The effects of harvest are HIGHLY variable. In high productivity streams - like many Driftless Area streams - increased harvest would probably be helpful. Yes, HELPFUL! Harvest can allow for the survivors to have more food and have their growth less limited by competition. In low productivity streams, harvest tends to have a larger impact. Another way to state it, compensatory mortality is lower in low productivity streams than in more productive streams where a larger proportion of the population can be harvested without negative effects.
However, the problem with harvest is the problem with most fisheries management issues - people are a huge part of the management equation. I will never forget the first and unfortunately not last Wisconsin DNR / Conservation Congress Spring Hearing that I attended. An older gentleman got up lamented the fact that you used to be able to keep your limit of trout and not one was under 20 inches. Ignoring for a minute that his memory is almost certainly "enhanced"; he is certainly ignoring that maybe that harvest had something to do with why we can no longer keep a limit of 20 inch trout. Smart harvest means harvesting the fishes that are abundant and easily replaced which largely means harvesting smaller fishes and letting the larger ones go to spawn and get larger.
Harvest should be a tool like so many others tools that fisheries management has such as habitat improvement, riparian zone protections, fish sanctuaries, etc. In my little corner of the Driftless Area, the heavily fished and well known Coon Creek (Timber Coulee) system (I'm not hotspotting or giving away any secrets here...) there is a diversity of harvest regulations ranging from catch and release to a ten fish, no size limit regulation. There are other streams that are "green" (5 fish, no size limit) and a stream that is "red" with a regulation of 5 fish under 12 inches. And I will tell you what - I see almost no difference in fish populations and maybe a bit of effect of harvest on size structure among these different regulations. Basically, on many Driftless Area streams there is not enough harvest to move the needle and the massive floods in recent years have barely moved the needle. And if they have, reducing the overall population a bit might lead to fish in better condition (a measure of how much they weigh at different lengths). It is also my experience that any effects of floods on trout populations in the area are rather short lived. More clean gravel means a massive year class the next year. Harvest really is the best way to manage these streams.
Now to be sure, none of this is saying that harvest does not have an affect on trout populations. We have overharvest of fishes ranging from Bluegill to the largest tuna species in the ocean. In much of Wisconsin where trout densities are not as high because the streams are not as fertile and well buffered; harvest certainly has an impact and in places. In some, too much of an impact. I am simply saying that in "my" streams, I have not seen harvest have much effect. I think in many parts of Wisconsin and the rest of the World, harvest and overharvest are important concerns. Here, I think our biggest "problem" with harvest is that fish over 14 inches are the ones I would like to see put back. However, they are more likely to be harvested than the 8 to 12 inch Brown Trout that are abundant in most Driftless streams. It is much harder to have 20 inch trout if you are not putting back 16 inch trout.
I will be the first to tell you, I am part of the problem. I very rarely keep fish, even from the ten fish, no size limit streams where they are BEGGING you to keep fish. I will when I plan for it because I do not like to eat fish that have not been handled correctly and I am often more interested in covering some ground than I am in being sure I can keep my catch cold and in good shape to eat. (For more on the topic of C&R and proper harvest)
The Take Home Message
Harvest is one tool that fisheries managers use to maintain sustainable and productive fisheries. Finding that "perfect balance" is nearly impossible. Many populations are overharvested and that leads to significant ecological issues. In other places - like my Driftless Area home waters - debates about harvest are overwrought and counterproductive. In fact, I would suggest that more smart harvest - keeping more sub-12 inch Brown Trout, releasing larger Brown Trout and any Brook Trout - would benefit our streams and may help produce more memorable and trophy trout.