Biologists talk about the continuum from species that have the capacity to grow exceedingly quickly under the right conditions - bacteria, mice, and many insects - and those that are very slow to change their populations due to a small number of offspring, long gestation times, and other factors that limit population growth rates - elephants, humans, and others. The first group we term r-selected species as their populations are a function of their growth rates - r in our population models. Catastrophic (density-independent) events that quickly and severely reduce their populations tend to be what limit their populations. This is basically what tends to keep r-selected species from over-running the world. K-selected species are those - like humans - whose populations tend to grow more slowly as they have fewer offspring, have longer gestation times, and controlled by carrying capacity (K). One one end of the spectrum are bacteria and cockroaches and on the other are Elephants and Blue Whales.
In population biology, r and K come from the exponential model and logistic models' most significant parameters. In exponential population growth, r, the rate at which the population grows does not change. This is a difficult concept for many because they confuse exponential rates with linear slopes but the more fiscally savvy understand it from compound interest. Put away a sum of money and when it grows by the interest rate, you have a much larger sum when you are ready to retire. Four percent of a million dollars is a pretty big number whereas 4% of $100 nets you about 4 dollars - somewhat more when compounded daily. Populations - like interest - do not grow linearly.
Logistic models are governed by K, the carrying capacity of a population. How fast the population grows to K is regulated by r, the rate at which it grows, but ultimately carrying capacity sets the limit on the population. A higher r just gets the population to carrying capacity (K) more quickly. Competition for food and other resources like space, mates, water; predation; and other biological interactions will set a limit on the population's density. To refresh our memories, exponential models are r-selected and density-independent whereas K-selected models are density-dependent and K-selected.
Source: Wikipedia - Survivorship Curves
Like the r and K selected species idea, survivorship curves are on a continuum. The curves above are rough approximations, idealized versions of survivorship. Take the lines for Humans, the one we are most familiar with. Unfortunately and rather sadly, the odds of a child making it from birth to age 1, its first birthday, are less than making it from its first birthday to its second birthday. Actuarial scientists, folks that are really good at math and generally work for insurance companies making very good money, tell us that your odds of getting in a car accident - particularly of your own doing - decrease quite significantly around your mid-20s so you saw, or will see if anyone under 25 is reading this, your car insurance rates plummet. Or try to buy life insurance in your 80s - the curve tells you why it is expensive or darn near impossible to purchase. The curves are generalizations but they do have a lot of truth - and math - built into them.
Type II survivorship curves have always been what fascinates me most. Think about being an individual of a species that has the same chance of dying at any time in their lives. That is one scary proposition to us type I species. For type III species, surviving to adulthood is like winning the lottery. The odds were very poor but once you win - reaching adulthood - you are likely to survive for a long time. Think oak or maple trees. Anyone that has mowed a late-summer or fall lawn has killed gazillions of developing maple trees. Exceedingly few make it to adulthood but once they do, they will likely survive another 50 or more years.
Type I species tend to be K-selected and with this survivorship curve tends to be species that have parental care, long gestation periods, relatively old age at first reproduction, and other longevity-related traits. On the r-selected / type III end, we tend to have species that produce an abundance of eggs, seeds, spores, or offspring but provide no parental care, little yolk or endosperm, or other things that help their seeds or offspring survive. These r-selected species are "fend for yourselves" but they put out so many offspring that few need to survive for the species to survive. Smaller prey species - mice, songbirds, insects, and other herbivores tend to be somewhere in the type II and III realms. Predators - lions, tigers, and bears (oh, my) - are classic type I species.
Holy shit, will he ever get to fish? I am getting there, I swear!
Fishes are a little weird when it comes to survivorship and fecundity (number of eggs, in the case of fishes). They break a lot of "rules". There are, of course, few rules in the biotic realm, just generalizations and exceptions to those "rules".
Bluegill (Lepomis macrochirus) are a perfect example. You might think that they are classic type III species being that they are food for nearly everything. And you would be sort of right but they do not exactly fit those expectations. Bluegill do not produce a crazy number of eggs and males guard the fry until they get large enough to leave the redd (nest). Similarly bass - largemouth and smallmouth - are a bit of an intermediate species, also showing nest guarding but also relatively low initial survival rates (but higher than if they did not nest guard).
Sturgeon, particularly the Lake Sturgeon (Acipenser fulvescens) that we in Wisconsin hear a lot about during the Lake Winnebago chain's spearing season, are another interesting exception to most rules. Female sturgeon first spawn at around 25 years old (pretty old for any fish) which is a rather K-selected trait but they produce a ton of eggs and provide no parental care - characters generally associated with r-selected species.
Trout are a little more straight forward and are further along the r-selected part of the continuum than the previous examples. Relative to a number of other fishes, trout eggs are relatively large for their body size - a trait generally associated with K-selection. But trout have no parental care - they put their eggs in a good spot where water flows through gravel keeping them well oxygenated and free of sediment - but their eggs need to survive for months (shorter for Rainbow Trout) before they hatch. During this incubation time, all sorts of density-independent events can happen - a careless wader, a lack of oxygen, anchor ice ripping the redd apart, or some other catastrophe. And for those eggs that do hatch and leave the redd, their chances of survival to adulthood is pretty low. They possess a typical type III survivorship. A young Brown Trout's greatest risk might be predation by larger Brown Trout.
As a type III survivorship species, trout have a pretty good chance of surviving once they get large. I think this is the best argument for selective harvest of the "intermediate" sized trout. Harvest those 9 to 12, maybe 14 inch Brown Trout and allow the larger ones to try to survive to 16, 18, or dare I say, that "magical" 20 inch mark. Fewer fish under 14 inches might also allow more room for growth of the larger fish. For Brook Trout, I have a hard time keeping any of them but if I did, I'd set the maximum size at 10, maybe 12 inches. A large, wild Brook or Brown Trout won a pretty significant "life lottery" that few - if any - of their brothers and sisters won.
Just my thoughts for another day...