Recruitment, growth, and mortality: three words that on the surface may not make an angler’s ears ring but buzz loudly in a fishery biologist’s. These three rate functions are important because they interact to shape fish populations. Recruitment and growth add, mortality subtracts. What results are characteristics of a fish stock—age structure, size structure, and density—at any point in time.
Sounds like a simple equation. Yet careers are made trying to understand the nuances of these connections, what affects the rate functions, how to best measure them, and how to most effectively manipulate them to achieve some desired end result.
In the realm of pike management, a common goal is to increase pike size in systems where larger pike once swam or were at least more common. Another is sustaining top-notch populations that currently exist. Although we don’t have all the answers yet, and it takes biological time to see results, the science is advancing on how to get the job done. Meanwhile, as progress inches forward, pike fisheries may be experiencing new challenges.
Pike Places
Habitat has a strong, if not the greatest, influence on pike reproductive success, recruitment (survival to adulthood), and growth rates. Meeting basic environmental requirements is a start, such as pike being a coolwater species with a specific thermal niche. Beyond that, what are the characteristics of waters that historically produce big pike over the long run?
That’s a question Peter Jacobson, fishery researcher with the Minnesota DNR, approached using long-term fishing contest data for pike caught in northwestern Minnesota lakes between 1924 and 1989. He asked: What factors are correlated with catches of trophy pike (those greater than 15 pounds) on a per acre basis? More big pike came from larger, deeper lakes with small littoral zones (area less than 15 feet deep). Biomasses of prey fishes—suckers, perch, and ciscoes—were also related factors.
Jacobson concluded that trophy pike management has the best chance of succeeding in lakes that maintain cool (68°F) water throughout summer, and that larger deeper lakes with ciscoes may show the most promise. Small pike, though, grew best inshallow fertile lakes with long growing seasons.
Along similar lines, Minnesota fishery biologists Rod Pierce and Cynthia Tomcko examined how characteristics of Minnesota lakes affected pike density and biomass. While the density of pike exceeding 14 inches was linked to the percentage of littoral habitat, lake area and shoreline length were more important factors for pike longer than 20 inches, the largest length group tested in the analysis.
All this points to connections between big pike and deeper, larger waters with thermal refuges during summer, along with coldwater forage like ciscoes. Make water too warm, eliminate thermal refuges, and pike become stressed, suffering longer periods of reduced growth in summer.
This is a phenomenon I observed while working on my dissertation research at South Dakota State University. Tracking seasonal growth of pike through monthly samples at Lake Thompson, a shallow windswept glacial lake in eastern South Dakota, growth ceased in summer while water temperature climbed through the mid- to upper-70°F range. The lake didn’t stratify so no thermocline-related thermal refuge existed. Annual growth rates were astoundingly fast overall, however, occurring during the cool- and cold-water periods. But pike tended to top out in the upper 30-inch range.
Limited production of trophies at Lake Thompson points to a live fast-die young lifestyle in warmer waters such as those at the southern fringe of the pike’s distribution. When I examined pike entries (15 pounds or 34 inches) in the South Dakota angler awards program during the years around the timeframe of my research, only 6 percent were caught from the state’s natural lakes, the majority taken from large deep reservoirs that thermally stratify.
Recruitment Factors
All this isn’t to say that smaller lakes don’t produce big pike. In fact, decades ago, 40-inchers extracted from select little gems around our Brainerd, Minnesota, headquarters and elsewhere in the pike belt weren’t uncommon. The habitat was there and harvest hadn’t yet taken its toll. Today, it’s rare but still possible to hook up with a legit gator at some of these locales. But for trophy pike on small waters, the best bets are private lakes, or remote waters with little pressure.
Unless a lake has been substantially impacted by shoreline development, or where adjacent wetlands used as spawning habitat have been destroyed, most pike lakes don’t have a lack of natural reproduction. Although habitat loss is a serious issue on some waters, a major limitation to managing for big pike in smaller lakes is that many of them are actually too good at producing pike—small ones.
Recruitment is a function of reproductive success and survival to adulthood. To successfully recruit, young fish need to grow and survive the gauntlet of predation, exploitation, and other sources of mortality. The higher the recruitment, though, the more numbers and usually pounds of pike in a lake. As pike biomass increases, competition rises, a factor that can slow growth. What can result is a stunted pike population—a large number of slow-growing pike with a high natural mortality rate. Here you might see upwards of 100 or more adult pike per acre, as opposed to less than five or so per acre in waters with low recruitment.
Recruitment tends to be related to lake size, with problems of high recruitment affecting mostly smaller waters with emergent vegetation covering a large proportion of the surface area. Larger lakes are rarely afflicted with stunted pike. Overpopulation has also been attributed to lack of appropriate size prey and overharvest of larger pike, or a combination of these factors.
Managing Exploitation
Anglers select for and harvest larger pike. In Minnesota, for instance, pike longer than about 24 inches make up a large portion of the harvest. “We’ve seen harvest rates as high as 46 percent for pike longer than 20 inches,” Pierce says. The result of this size selectivity is that numbers of bigger pike have suffered.
In a set of north-central Minnesota lakes, he found that about 9 to 10 pike longer than 14 inches exist per acre of water on average, compared to only a half of a pike per acre for fish longer than 24 inches. “Besides density, we can also look at production rates—the amount of tissue produced on an annual basis—to provide some indication of how much harvest a fishery can sustain,” he says. “It’s really low for the largest pike in a population. The second and third growth years made up 60 to 87 percent of the annual production, while pike age 6 and older averaged only 4 percent.
