“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.
