FISH GROWTH, TROPHIES AND THE ROLE OF HARVEST
In a Hooked article published a few months ago, we discussed the basics of fish growth. We explained how being cold-blooded meant that most north-temperate fish species exhibit most of their growth when the water is warm, and that winter tends to be a non-growth period.
We talked about how biologists age fish, the problems (errors) associated with trying to age older fish, and funding constraints which frequently preclude a good understanding of the “top end” of fish growth curves in most lakes and rivers. Finally, we touched on factors that can influence fish growth such as latitude, food availability and energetic costs of food acquisition and predator avoidance. In Part 2, we use a simple walleye dataset to elaborate on a key concept: density dependent growth.
Density dependent processes occur when the growth of a population is regulated by the density of the population. Clear as mud? Another way of looking at density dependent growth in the context of fish is simply that growth rates decline if there are too many fish.
A WELL KNOWN LOCAL EXAMPLE!
Crowduck Lake, Manitoba is the largest lake (55.6 km2) in Whiteshell Provincial Park. It was stocked with walleye during the late 1980s. Catch-and-release regulations (for walleye only) were also implemented. The walleye fishery took off and before long anglers began registering Master Anglers. I queried Travel Manitoba’s database to produce the figure shown below. In 1991 (shortly after stocking occurred), only a couple of Master Angler sized walleye were registered. By 1995, over 150 were registered.
Annual registrations peaked in 1997 (over 640!). Then as quickly as the quantity of trophies being registered rose, they started to taper off. Incidentally, I had the pleasure of fishing Crowduck for the first time in 2002. While we caught over 100 walleye per person on an amazing day trip via Crow Duck Lake Camp, we did not catch any Master Anglers.
Now understanding what is revealed via Travel Manitoba’s unique data archive, it probably was not due to our googan fishing skills! Rather, the Master Angler records data suggest that there simply were not many large walleyes alive and swimming at that time. It also bears mention that I have never seen as many skinny, malnourished walleye as I did in Crowduck during 2002.
Crowduck Lake Master Angler walleye catches over time, based on data queried from the Travel Manitoba Master Angler Record Book. It should be noted that variation in angling pressure over time could somewhat bias the interpretation of the Master Angler database.
Insufficient funding and staffing has precluded Manitoba Fisheries Branch from doing widespread fisheries monitoring throughout the province for the better part of two decades. The lack of a robust biological dataset for the Crowduck Lake population leaves some room for doubt about the reasons for the observed decline in trophy walleye abundance.
However, the logical explanation is that by 2002 (when I fished the lake) the suddenly large Walleye population had consumed the bulk of the available forage. Presumably, density dependence had kicked in (hard) by about 2000. The walleye had eaten themselves out of house and home!
NOTHING LEFT TO EAT
Since 2003, only a relative few Master Angler walleye have been registered from Crowduck Lake – on average only 5 per year between 2003 and 2019. Although we cannot say for certain since monitoring data is lacking, it stands to reason that growth rates of females (males typically do not attain Master Angler size) declined rather drastically relative to patterns observed in the 1990s. As a result, the vast majority of females simply stopped attaining Master Angler size prior to succumbing to natural morality.
CHANGES TO REGULATIONS NEEDED
I feel it is extremely unlikely that reduced fishing pressure is confounding the interpretation, as anglers from all over North America continue to flock to Crowduck Lake. The experience provided by Crow Duck Lake Camp is A+, and the walleye fishing remains outstanding, albeit for mid-sized fish (and not trophies). Now that Catch-and-release regulations have been removed and anglers are allowed to keep two Walleye less than 17.5”, it will be very interesting to revisit the Crowduck Lake Master Angler records in another 20 years and see if trophy walleye catches increase.
Hypothetical (not based on any particular empirical data set) walleye length-at-age relationships (females only), at differing population densities. As walleye density increases and forage becomes limiting, fish grow slower. These curves reflect hypothetical population averages; individual fish may grow somewhat faster or slower. Based on the curves presented, female walleye at low density attain lengths of 28” around age 11. Female walleye at moderate density do not attain 28” until age 15, by which time very few individuals remain.
NO CHANCE FOR A TROPHY FISH
Under a high- density walleye situation, essentially none attain 28”. Biologists use the term “stunting” to describe populations that become dominated by individuals characterized by high density growth trajectories, and as a result, very few fish that achieve typical top-end size possible for the species in lower density environments.
WHERE THINGS GET MESSY!
Because fish like walleye can exhibit 15 year lifespans (and longer in some populations), understanding the dynamics of growth rate and optimizing harvest regulations to ensure fisheries management objectives (such as presence of trophy sized fish) will be met over the long-term can be difficult. Significant quantities of large individuals (that attained their current size via rapid growth trajectories) can be present in the population, but it may not be readily apparent that younger fish in the population are now growing much slower than the previous generation.
A DECADE LATER THE RESULTS MANIFEST THEMSELVES
A decade later, this slow growth rate will manifest in the rarity of trophy sized fish, at which point the timeline for correcting the issue is on the order of generational time (e.g. approximately 10 years for walleye). This is particularly likely when population size increases rapidly (e.g. following stocking, as seen with Crowduck Lake). This type of scenario makes it challenging for fisheries managers operating in a data-poor environment to gauge if intervention (e.g. a change in regulations that promotes a desired outcome) may be warranted.
NO DATA AVAILABLE
A single biological survey of a given lake can produce length-at-age estimates as well as estimates of walleye abundance and fish community composition that provides some insight. Without the perspective of a long-term monitoring dataset it becomes problematic to determine, for example, what walleye that exhibit an average length of 13” at age four actually means.
That metric could simply be driven by sub-optimal geomorphologic characteristics of the lake, and a less than ideal forage base.
Conversely, if the manager knew that in the 1990s, age 4 walleye in the same lake averaged 16”, it would be clear that growth rates are much lower than they used to be. If forage quantities also used to be much higher in the lake, it might be worth considering a change in regulations to increase walleye harvest.
The goal would be to reduce their abundance to a level more likely to produce rapid growing fish that might ultimately achieve trophy size. Of course, the size versus numbers prioritization is largely in the eye of the beholder– having a glut of mid-sized walleye is not necessarily a bad thing. Even without many trophy sized fish present, Crowduck Lake is a fantastic experience for many anglers.
Perhaps more contentious is the situation wherein a species whose primary value is in relation to catch-and-release angling of large individuals – smallmouth bass. Manitoban’s generally have the mindset that smallmouth bass are solely a sport fish, which I suspect relates to the perceptual legacy of historical catch-and-release angling regulations. Anecdotally, it appears that smallmouth bass have become so abundant in some smaller Manitoba lakes that the abundance of trophy sized fish is not what it once was.
PART THREE ON THE WAY
In Part 3, we will explore the state of several lakes stocked with smallmouth bass and delve further into the logic behind a strategy that hopefully (after reading about Crowduck Lake) will not seem counter intuitive. In a nutshell, if anglers want many of the provinces’ trophy smallmouth lakes to persist as such, harvest of smaller fish may need to occur so as to encourage growth rates that ultimately result in the production of significant quantities of Master Angler sized fish.
FISHERIES MANAGEMENT OBJECTIVES!
Fisheries management objectives are goals that are strived for in the context of aquatic systems used by humans. Conservation/sustainability typically ranks very highly, but not always (e.g. stocked trout fisheries which do not reproduce naturally). Other possible objectives that fisheries managers consider include food production (harvest) and the generation of economic wealth (as high-quality angling can drive tourism).
REALISTIC EXPECTATIONS!
Reducing high-density walleye populations to more moderate levels of abundance via harvest will encourage more rapid rates of growth, and therefore increased production of trophy sized fish can be expected. However, it would be unrealistic to expect reductions in abundance to yield growth rates characteristic of the initial post-stocking “boom” period, because 1) significant quantities of walleye still remain and 2) the forage base is unlikely to recover to pre-stocking levels of abundance.