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.

“That equates to an average production of only a tenth of a pound per acre per year for pike age 6 and older. Theoretically if you had a 100 acre lake and removed a 10-pound pike, you’d use up the entire production of large pike for a full year,” he says.

From the perspective of angling effort, Ontario biologists Tom Mosindy, Walter Momot, and Peter Colby found that as little as 1.2 hours per hectare of fishing effort removed 50 percent of the annual pike production in a Canadian shield lake. More fertile waters can produce more pounds of pike per acre and withstand more harvest, but these numbers show how vulnerable the largest pike in a system are.

Length-limits may be the most effective regulation tool for managing harvest and growing larger pike, and the correct choice of a length limit depends largely on recruitment. Where recruitment is high and there are high densities of slow growing pike, protected slot-length limits allow harvest of small pike. The intent: thin the numbers of smaller pike to improve growth and size structure of the remaining pike, while protecting larger fish within the slot. The problem: anglers generally don’t harvest small pike enough to show any effect. Improvements, if any, tend to be due to some pike making it through the growth bottleneck into the protected slot.

High minimum-length limits are a better choice for preserving or restoring trophy pike on waters with low recruitment, low density, and good growth potential. High slots, maximum-length limits, and catch-and-release are other options to restore big pike or to preserve unexploited trophy fisheries once they’re opened to fishing, such as the gator factories in the Far North.

Since 2003, over 100 lakes in Minnesota have received special length-limit regulations, one of three in a toolbox of choices based on pike population characteristics: a 24- to 36-inch protected slot, 30-inch minimum, and a 40-inch minimum. Those lakes are under evaluation for 10 years.

Pierce, however, has completed an evaluation of experimental regulations that were imposed between 1989 and 1997, lasting 9 to 15 years. “Those included some slot limits (20- to 30-inch or 22- to 30-inch), maximum-length limits (20-, 22- or 24-inch), and a few 30-inch minimums. Although the length regulations did not work in every lake, the bottom line overall was that, statewide, length limits had a relatively large effect on size structure compared to reference lakes. The strongest effects seemed to be from maximum- and minimum-length limits, whereas the slot limits had more mixed results, probably due to the modest range of lengths that we protected.

“In all, length limits seem to be one of the most promising tools we have for managing pike populations. One of the more interesting results is that we didn’t detect any consistent reductions in pike numbers with the length limits. We had fully expected that improving size structure would result in lower density, and that hasn’t happened yet,” he says.

Pressure Cooker

While length limits make headway, a warming climate may be imposing new challenges to pike. Warning shots are already being sent across the bow, as researchers studying the effects of climate change on fish are seeing responses that run hand-in-hand with climate trends.

“The changing climate is affecting aquatic environments, fish, and fisheries,” says Dr. John Casselman, adjunct professor at Queen’s University and former Senior Scientist with the Ontario Ministry of Natural Resources. “Environmental conditions are changing as are baseline conditions. In the last 20 years, for example, we’ve seen a rise of 1.5°C in the temperature of Ontario lakes. If climate warming continues, our models predict a rise in summer water temperature of 2°C in the next 20 years, 3°C over the next 30. There’s an invisible shift going on and for pike that means a northward shift.”

Casselman says that with increasing midsummer temperatures, recruitment has increased in warmwater fish and decreased in coolwater fish in Ontario waters. “If waters warm an additional 1°C in Ontario lakes, we predict pike recruitment will decrease 2.4 times, or 240 percent. If the temperature warms 2°C, recruitment will decrease almost 18 times.

“What’s critical, too,” Casselman says, “is how climate change affects coldwater fish because of the strong connections between trophy pike and coldwater species like ciscoes. Warming negatively affects spawning. With a 1°C rise in water temperature we calculate a 1.5-fold decrease in recruitment of coldwater fishes, a 3°C rise reduces recruitment 20-fold.

“There are examples of lakes in Ontario that have never been commercially fished,” he notes, “waters that historically had healthy populations of coldwater species such as whitefish. Now all you see are large adults and they’re all 30 years old. Where are the younger age groups?” Casselman suggests that warming water has already taken its toll on recruitment at these locales.

Cisco Trends

In Minnesota, ciscoes are an important forage species in about 650 lakes, more widespread than most people realize, says Peter Jacobson. Based on over 3,000 netting assessments from 1947 to 2007, however, he’s seeing a decline in cisco abundance.

“When we look at Minnesota statewide averages, there’s been a significant decline since around 1975, and it appears to be the result of climate change. It’s happening on a number of lakes, and in some lakes ciscoes no longer exist.” Jacobson says populations that are taking a hit are mostly in lakes with marginal-habitat at the southern edge of their range in the state.

Over past decades, Minnesota has been experiencing climate trends, Jacobson says, particularly warmer winters and warmer nights during summer, setting up conditions for a longer growing season. Combine that with cisco facts: they require water less than about 68°F and dissolved oxygen levels above about 4 parts per million.

A longer growing season means a longer period of thermal stratification, which can be detrimental to coldwater species that rely on a limited layer of cool, oxygenated water near the thermocline. “When a lake remains stratified longer, oxygen depletion in the hypolimnion, the bottom layer below the thermocline, becomes more likely. As an anoxic hypolimnion expands upward, a warming epilimnion pushes downward, so the livable habitat zone for ciscoes shrinks and in the worst case disappears,” Jacobson explains. When thermal stress gets too high, die-offs occur.

How well do ciscoes adhere to their thermal niche? Andy Carlson, Minnesota DNR researcher, is using acoustic tags to track depth and temperature use of ciscoes in a thermally “marginal” Minnesota lake. In his study lake, Carlson says suitable conditions for ciscoes were restricted to a layer of water only about 2 to 3 feet thick, and that’s the depth range that the tagged ciscoes used. “Some individuals venture above and below that layer at times, but mostly they are sandwiched into that narrow zone,” Carlson says.

Further warming can squeeze ciscoes into an ever-shrinking zone. Extreme warming events several years in a row can have catastrophic consequences by eliminating a cisco population altogether.

Critical Habitats

“How declines in cisco abundance will affect trophy pike production is definitely a concern, especially on these marginal lakes.” Jacobson says. “What we consider the good pike lakes, those with deep, cold, clear water, tend to have good oxygen levels in the hypolimnion and should withstand the brunt of the expected levels of warming.

“In prime pike lakes we’re worried mostly about changes in water quality. Cultural eutrophication— human induced nutrient loading—can cause a hypolimnion to expand and become more anoxic, turning a good lake into a marginal one.

“Shoreline development is another concern. And beyond the shoreline, we should be protecting watersheds that drain into high-quality pike lakes. We may not be able to reverse climate trends and their direct impact on waters and fish, but through watershed protection we can exert some level of control,” Jacobson says.

Casselman explains that it’s not only the direct effects of warming that affect pike. “When water warms, evaporation also increases. We find that for every 1°C rise in water temperature there’s a 6 percent increase in evaporation. This affects runoff, wetland flooding, spawning habitat, and eventually pike recruitment. When recruitment declines, fewer pike are available to reach advanced age in the face of fishing mortality, which may mean fewer trophy pike.”

There are remarkable signs that pike may be adapting to changing conditions, Casselman says. “On the St. Lawrence River, for instance, pike are spawning deeper and earlier. Instead of spawning in 3 feet of water they are spawning in 6 feet, adapting to springs with weak runoff and little rain.”

That pike are adapting on these time scales is incredible, he says, but there likely isn’t going to be time for natural selection to keep up with the pace of the changing environment. “Pike aren’t going to fix things. We’re the ones that will have to adapt.”