FISH GROWTH, TROPHIES AND THE ROLE OF HARVEST
Growth of fish is, at least on the surface, a relatively simple concept – as fish age, they become increasingly large. To put that in scientific terms, fish exhibit “indeterminate” growth, meaning that they don’t stop growing at a certain point in their life (like birds or mammals, including humans). As a result, it is often generalized that the oldest fish of a certain species are the largest – the trophies that many anglers invest time, money and effort into catching.
MANY VARIABLES TO HOW FISH GROW
However, there can be large variation in growth rate and top-end size among different regions (in North America, fish generally grow faster in southern versus northern latitudes), among different waterbodies located near to each other, and even within a waterbody! There are some very interesting growth-related trends currently playing out in the waters of Manitoba and Northwest Ontario, and over the next few articles for Hooked these will be explored. But first, the basics of fish growth.
THE SCIENCE OF FISH
Fish are poikilotherms (cold blooded), meaning that they can’t regulate their body temperature. Therefore, a fish’s metabolism and capacity for growth is most strongly influenced by the ambient temperature of its surrounding environment. In north-temperate waters, the vast majority of growth for species like Walleye, Bass, Perch and Lake Sturgeon happens between spring and fall (warm water period), and relatively little happens during winter (cold water). A great deal can be learned about growth by studying fish reared in hatcheries, where water temperature and food availability can be controlled.
At Manitoba Hydro’s Grand Rapids Fish Hatchery, Lake Sturgeon are often reared over their first winter to maximize post-stocking survival. During overwinter residence in the hatchery, the transition point between growth and non-growth for Lake Sturgeon appears to be ~10°C. I recently learned from Dr. Cheryl Klassen (who is involved with the fish production program at the hatchery) that even though plenty of food is always provided over winter, if water temperatures are maintained at ~4°C, the Lake Sturgeon feed at a slow rate and individuals are essentially the same size come May (when water temperatures increase above 10°C) as they were the previous November.
HOW DO YOU AGE FISH?
Fish do not carry birth certificates, so the method biologists most often use to age fish and examine growth involves detailed examination of hard-structures such as fin rays, scales or otoliths (calcium based structures found in a fish’s inner ear). The premise is that in north-temperate lakes and rivers, alternating dark and light bands deposited reflect periods of growth (spring to fall) and non-growth (winter), respectively. The light bands (annuli) are typically counted to determine how old the fish is. In northern latitudes, these methods tend to be fairly robust for young fish, which can likely be attributed to the pronounced seasonality we experience. Long harsh winters tend to result in growth cessation, which leads to pronounced banding (easy to count rings). It can be more difficult to assign accurate ages in southern latitudes.
STURGEON HAVE SURVIVED THE AGES FOR A REASON
On occasion, the Lake Sturgeon reared over winter in the Grand Rapids Fish Hatchery have been exposed to temperatures of ~15°C, which is much greater than the 0°C low they would have experienced naturally in ice-covered rivers that do not stratify. These fish grew rapidly, and were “large-at-age” (relative to wild-spawned Lake Sturgeon of the same age) by the time they were released in spring (as one year olds). Perhaps not surprisingly, these fish also exhibit weak/absent “first annuli” in their growth chronologies when biologists have examined fin rays following recapture in the wild many years post-release.
AGING IS NOT AN EXACT SCIENCE
Ageing of older fish tends to be much more difficult, and validation studies for a variety of species confirm that biologists often cannot accurately assign ages to older fish using hard-structure examination. The presence of false annuli (rings deposited more frequently than normal), hard-structure compression (annuli so tightly spaced that they are hard to discriminate between), and the allocation of energy to spawning all pose complications. As such, growth rates for older fish are sometimes best calculated using incremental methods – for a 5-year old Walleye that was 17” when tagged, we could calculate it had grown 7” in 8 years when it is recaptured as a 24” fish at age-13.
MORE MONEY NEEDED FOR RESEARCH
Unfortunately, most fisheries programs are understaffed and operating on shoe-string budgets. Essentially, not enough resources are available to conduct the required mark-recapture tagging type of studies across the diversity of lakes we are blessed with in Canada, meaning that the top end of fish growth curves in a given lake is often questionable.
FACTORS THAT INFLUENCE GROWTH
In addition to temperature, food availability and the energetic costs of both food acquisition and predator avoidance can strongly influence growth. Fish that inhabit flowing waters may need to expend energy to hold position and pick off small organisms as they drift passively downstream. Higher flows mean more energy is consumed to hold position, which yields slower growth.
ON THE RUN
Similarly, fish that are constantly on the run from predators (e.g., small Walleye in a lake with lots of Northern Pike) must also expend energy or risk being eaten, which means that not all of the energy they accumulate can be allocated to growth.
Macroinvertebrates (e.g., crayfish, snails, insect larvae) dominate the forage base of young fish, and distributions of these organisms can be influenced by numerous physical factors, including water depth, water clarity, and substrate (bottom-type). No two waterbodies are the same in terms of their physical attributes, meaning there will be inherent differences that drive variation among their food webs.
NOT DONE YET!
As if all the aforementioned factors were not complicated enough, fish growth can also be influenced by other fish. When there are too many fish of the same species (or even two different species which compete with each other for food) relative to the amount of food available, growth rates often decline. If the number of fish is later reduced (e.g., if harvest increases in response to increased numbers of fish), growth rates often rebound.