Garrison Keillor, the famed storyteller and self-proclaimed “resident” of the fictional town of Lake Wobegon, Minnesota, commented on the ferocious nature of deer flies that attacked him while fishing one summer. “Bug lotion doesn’t work,” he remarked. “Crucifixes help - but you have to hit them real hard!” While deer flies are of concern to Minnesotans, we Alaskans have our own version of large, aerial predators … those bloodthirsty mosquitoes! For this reason, we can’t help but find a bit of sordid pleasure when the tables are turned on one of these ruthless little pests and it relinquishes its role as predator and becomes the prey! When a mosquito buzzes down to the surface of a stocked lake, it is unaware that predatory fish are lurking and ready to strike, albeit with one caveat: the fish must be hungry and actively foraging. Foraging is the behavior of searching out and consuming food. But animals don’t forage at random. The question of what drives foraging in fish is something that has intrigued scientists (and fishermen) for years.
There are few things more frustrating than spotting a nice trout or grayling in the water column, but not being able to interest it in a lure or bait. As equally puzzling (but far more enjoyable), are the times when the fish are biting at everything, and you feel as though you could drag your car keys through the water on a string and get a bite. So what is it, exactly, that drives appetite in fish and determines their interest in food? The answer is somewhat complex, as it involves both their physiology and their environment.
Why Won’t They Bite?
Several months ago on a mild, sunny afternoon, I sat on the bank of Kepler Lake in the Matanuska Valley, waiting for the tip of my fishing rod to bend. Dry flies hadn’t produced any hook-ups that day, and my attempts at casting small spinners with a light tackle rod weren’t paying dividends. I resorted to bottom-fishing a halved nightcrawler in hopes that the scent might interest something.
The fish were definitely there. I had spotted a few trout on another fisherman’s stringer, caught earlier in the day. Added to this was knowledge that Alaska Department of Fish and Game (ADF&G) reported stocking the Kepler-Bradley Lakes with thousands of rainbow trout and grayling for the last several summers. My mind ran through the checklist of what could be deterring the fish from biting. Quiet location? Check. Mild temperatures? Check. Multiple fishing methods and techniques attempted? Check. What was going on? What was suppressing the appetite of these fish?
Feeding and growth of fish can be regulated by both intrinsic and extrinsic factors. Intrinsic factors are internal molecules, processes or mechanisms that mediate or influence feeding. These include things like hormones, endogenous rhythms or cycles, nutritional state, reproductive status, or gene expression. Extrinsic factors are environmental conditions, cues or influences, including daylength, water temperature, habitat features, and the presence of predators or prey items in the area.
The Intrinsic Factors
Fishes’ appetites and feeding activities are mediated by a suite of hormones produced in their brains and peripheral organs. In fact, there are twenty-three different known endocrine factors that help to regulate appetite in fish[1]. These include appetite stimulators, termed orexigenic factors (derived from the Greek, “orexia”, meaning “to eat”), as well as appetite inhibitors known as anorexigenic factors, a term similar to the familiar word, “anorexia.” Changes in fishes’ levels of these appetite-regulating hormones and receptors, as well as gene expression, are linked to appetite and feeding behavior in fish.
The hormones regulating a fish’s appetite are actually produced in many different areas of its body, including the brain, pituitary, liver, pancreas, GI tract, and others. Some of its orexigenic hormones, such as neuropeptide Y (NPY) and orexin (OX), are produced in the brain, while ghrelin is produced in the stomach. Neuropeptide Y stimulates appetite and feeding and also upregulates the secretion of growth hormone (GH), which leads to increased growth of fish. Increases in GH also stimulate more foraging activity. Anorexigenic factors include cocaine-and amphetamine-regulated transcript (CART) and melanin-concentrating hormone (MCH), which are produced in the fish’s brain; leptin, which is produced in its liver; and cholecystokinin (CCK), produced in the gut.
In fish, the expression of appetite-related peptides and feeding behavior can be affected by the anticipation of feeding, the actual ingestion of food, circulating metabolite levels in the blood and food deprivation. For instance, the anticipation or consumption of a predatory fish’s first meal of the day, whether a mosquito or smolt, increases the levels of gene expression of orexigenic appetite regulators, such as NPY or OX. Conversely, gene expression levels of anorexigenic appetite regulators CCK and CART decrease following feeding. Food deprivation or starvation in a fish usually leads to the upregulation of orexigenic factors (NPY, OX, and ghrelin) and the downregulation of anorexigenic factors (CART and CCK). This, in turn, leads to increased foraging activity – and from the fisherman’s perspective - a greater likelihood that a fish will strike.
Gender, reproductive status and sex steroid levels of fish also have the potential to influence feeding behavior and growth. For instance, fishes’ appetite hormones appear to be linked to activities of some reproductive hormones, such as gonadotropin releasing hormone (GnRH), which is synthesized in specialized neurons in the brain.
Endogenous feeding rhythms also exist in fish. Circadian and circannual rhythms can affect the timing and amount of food ingested, as well as the efficiency at which it is assimilated. Furthermore, some hormones known to be correlated with feeding activity (e.g., thyroid hormone, cortisol, and GH) can be secreted in similar daily or annual rhythms. The same inherent rhythms are present in humans, as some claim they feel hungry or become tired at the same times every day; others report feeling more active in summer, but sluggish in winter.
The Extrinsic Factors
Fish inhabiting lakes in Alaska must endure a strongly seasonal environment, including daylength extremes in summer and winter and long bouts of ice cover and cold water temperatures in winter. These and other extrinsic factors can influence fishes’ swimming activity, metabolism, growth and levels of their appetite hormone. Daylength, for example, can influence plasma GH, thyroid hormone, cortisol, melatonin and sex steroid levels of fish. It’s no wonder that the crack of dawn and the early evening are generally believed to be the times when the fish are biting. Although effects of extrinsic factors on fish physiology have been determined primarily through captive studies, it can be reasonably assumed that they have the potential to impact feeding in wild populations.
An increase in water temperature tends to increase feeding. However, on excessively warm days when food is not optimally available in shallow water, fish can head to deeper water. However, dissolved oxygen (D.O.) content is an important factor that can change dramatically with lake depth. Deeper areas can sometimes have reduced D.O. levels. It can be helpful to have information about the depth and features of a lake to try and determine where fish might be hiding out. Fortunately, the ADF&G has invested the time and money into developing bathymetric maps for many roadside lakes, including Kepler-Bradley.
Bathymetry (derived from the Greek, “bathus,” meaning “deep,” and “metron,” meaning “measure”) refers to the measurement of lake depth. Classic bathymetry methods involved hand-mapping lake depths by dropping a weighted line throughout different sections of a lake. Today, it is done through the use of sonar from a boat, with data fed to a computer. Bathymetric maps help agencies to estimate lake volumes and calculate carrying capacities for fish. They can also provide fishermen with valuable information about fish habitat and possible locations. This can be helpful in understanding where fish might be located, depending on the temperature, water quality, the time of year and the presence or absence of potential prey or predators.
Now Go and Fish
For anglers with access to a canoe or rowboat, knowing where drop offs and the deepest parts of the lake are before launching can provide valuable information and save time. On Kepler Lake, canoes, rowboats and gear are available for rent at the channel that connects it with Bradley Lake. An assessment of the bathymetric map for these waterbodies shows Kepler having the greater maximum depth. It sinks quickly from 30 to 74 feet, but only out towards the western end of the lake. Bradley Lake has a maximum depth of only about 30 feet deep. Deeper areas with drop-offs are often good places to search different sections of a water column when a fish finder is not an option. But don’t completely neglect the shallows, especially the points off the edges of coves.
There is still much to be learned about the factors that influence foraging behavior in fish while in their natural habitat. When the fish aren’t biting, pondering the internal mechanisms and external factors regulating fish appetite and feeding can be a reasonably entertaining pastime. However, when I’ve hooked into a nice rainbow or grayling on my 4wt fly rod, the science of foraging is honestly the furthest thing from my mind. I want to battle the fish! But I do believe that the knowledge of what factors influence or drive fish to search for food has helped me to become a better angler. It’s also helped me to have a greater appreciation for how complex and interesting these fantastic aquatic creatures truly are.
Garrison Keillor, the famed storyteller and self-proclaimed “resident” of the fictional town of Lake Wobegon, Minnesota, commented on the ferocious nature of deer flies that attacked him while fishing one summer. “Bug lotion doesn’t work,” he remarked. “Crucifixes help - but you have to hit them real hard!” While deer flies are of concern to Minnesotans, we Alaskans have our own version of large, aerial predators … those bloodthirsty mosquitoes! For this reason, we can’t help but find a bit of sordid pleasure when the tables are turned on one of these ruthless little pests and it relinquishes its role as predator and becomes the prey! When a mosquito buzzes down to the surface of a stocked lake, it is unaware that predatory fish are lurking and ready to strike, albeit with one caveat: the fish must be hungry and actively foraging. Foraging is the behavior of searching out and consuming food. But animals don’t forage at random. The question of what drives foraging in fish is something that has intrigued scientists (and fishermen) for years.
There are few things more frustrating than spotting a nice trout or grayling in the water column, but not being able to interest it in a lure or bait. As equally puzzling (but far more enjoyable), are the times when the fish are biting at everything, and you feel as though you could drag your car keys through the water on a string and get a bite. So what is it, exactly, that drives appetite in fish and determines their interest in food? The answer is somewhat complex, as it involves both their physiology and their environment.
Why Won’t They Bite?
Several months ago on a mild, sunny afternoon, I sat on the bank of Kepler Lake in the Matanuska Valley, waiting for the tip of my fishing rod to bend. Dry flies hadn’t produced any hook-ups that day, and my attempts at casting small spinners with a light tackle rod weren’t paying dividends. I resorted to bottom-fishing a halved nightcrawler in hopes that the scent might interest something.
The fish were definitely there. I had spotted a few trout on another fisherman’s stringer, caught earlier in the day. Added to this was knowledge that Alaska Department of Fish and Game (ADF&G) reported stocking the Kepler-Bradley Lakes with thousands of rainbow trout and grayling for the last several summers. My mind ran through the checklist of what could be deterring the fish from biting. Quiet location? Check. Mild temperatures? Check. Multiple fishing methods and techniques attempted? Check. What was going on? What was suppressing the appetite of these fish?
Feeding and growth of fish can be regulated by both intrinsic and extrinsic factors. Intrinsic factors are internal molecules, processes or mechanisms that mediate or influence feeding. These include things like hormones, endogenous rhythms or cycles, nutritional state, reproductive status, or gene expression. Extrinsic factors are environmental conditions, cues or influences, including daylength, water temperature, habitat features, and the presence of predators or prey items in the area.
The Intrinsic Factors
Fishes’ appetites and feeding activities are mediated by a suite of hormones produced in their brains and peripheral organs. In fact, there are twenty-three different known endocrine factors that help to regulate appetite in fish[1]. These include appetite stimulators, termed orexigenic factors (derived from the Greek, “orexia”, meaning “to eat”), as well as appetite inhibitors known as anorexigenic factors, a term similar to the familiar word, “anorexia.” Changes in fishes’ levels of these appetite-regulating hormones and receptors, as well as gene expression, are linked to appetite and feeding behavior in fish.
The hormones regulating a fish’s appetite are actually produced in many different areas of its body, including the brain, pituitary, liver, pancreas, GI tract, and others. Some of its orexigenic hormones, such as neuropeptide Y (NPY) and orexin (OX), are produced in the brain, while ghrelin is produced in the stomach. Neuropeptide Y stimulates appetite and feeding and also upregulates the secretion of growth hormone (GH), which leads to increased growth of fish. Increases in GH also stimulate more foraging activity. Anorexigenic factors include cocaine-and amphetamine-regulated transcript (CART) and melanin-concentrating hormone (MCH), which are produced in the fish’s brain; leptin, which is produced in its liver; and cholecystokinin (CCK), produced in the gut.
In fish, the expression of appetite-related peptides and feeding behavior can be affected by the anticipation of feeding, the actual ingestion of food, circulating metabolite levels in the blood and food deprivation. For instance, the anticipation or consumption of a predatory fish’s first meal of the day, whether a mosquito or smolt, increases the levels of gene expression of orexigenic appetite regulators, such as NPY or OX. Conversely, gene expression levels of anorexigenic appetite regulators CCK and CART decrease following feeding. Food deprivation or starvation in a fish usually leads to the upregulation of orexigenic factors (NPY, OX, and ghrelin) and the downregulation of anorexigenic factors (CART and CCK). This, in turn, leads to increased foraging activity – and from the fisherman’s perspective - a greater likelihood that a fish will strike.
Gender, reproductive status and sex steroid levels of fish also have the potential to influence feeding behavior and growth. For instance, fishes’ appetite hormones appear to be linked to activities of some reproductive hormones, such as gonadotropin releasing hormone (GnRH), which is synthesized in specialized neurons in the brain.
Endogenous feeding rhythms also exist in fish. Circadian and circannual rhythms can affect the timing and amount of food ingested, as well as the efficiency at which it is assimilated. Furthermore, some hormones known to be correlated with feeding activity (e.g., thyroid hormone, cortisol, and GH) can be secreted in similar daily or annual rhythms. The same inherent rhythms are present in humans, as some claim they feel hungry or become tired at the same times every day; others report feeling more active in summer, but sluggish in winter.
The Extrinsic Factors
Fish inhabiting lakes in Alaska must endure a strongly seasonal environment, including daylength extremes in summer and winter and long bouts of ice cover and cold water temperatures in winter. These and other extrinsic factors can influence fishes’ swimming activity, metabolism, growth and levels of their appetite hormone. Daylength, for example, can influence plasma GH, thyroid hormone, cortisol, melatonin and sex steroid levels of fish. It’s no wonder that the crack of dawn and the early evening are generally believed to be the times when the fish are biting. Although effects of extrinsic factors on fish physiology have been determined primarily through captive studies, it can be reasonably assumed that they have the potential to impact feeding in wild populations.
An increase in water temperature tends to increase feeding. However, on excessively warm days when food is not optimally available in shallow water, fish can head to deeper water. However, dissolved oxygen (D.O.) content is an important factor that can change dramatically with lake depth. Deeper areas can sometimes have reduced D.O. levels. It can be helpful to have information about the depth and features of a lake to try and determine where fish might be hiding out. Fortunately, the ADF&G has invested the time and money into developing bathymetric maps for many roadside lakes, including Kepler-Bradley.
Bathymetry (derived from the Greek, “bathus,” meaning “deep,” and “metron,” meaning “measure”) refers to the measurement of lake depth. Classic bathymetry methods involved hand-mapping lake depths by dropping a weighted line throughout different sections of a lake. Today, it is done through the use of sonar from a boat, with data fed to a computer. Bathymetric maps help agencies to estimate lake volumes and calculate carrying capacities for fish. They can also provide fishermen with valuable information about fish habitat and possible locations. This can be helpful in understanding where fish might be located, depending on the temperature, water quality, the time of year and the presence or absence of potential prey or predators.
Now Go and Fish
For anglers with access to a canoe or rowboat, knowing where drop offs and the deepest parts of the lake are before launching can provide valuable information and save time. On Kepler Lake, canoes, rowboats and gear are available for rent at the channel that connects it with Bradley Lake. An assessment of the bathymetric map for these waterbodies shows Kepler having the greater maximum depth. It sinks quickly from 30 to 74 feet, but only out towards the western end of the lake. Bradley Lake has a maximum depth of only about 30 feet deep. Deeper areas with drop-offs are often good places to search different sections of a water column when a fish finder is not an option. But don’t completely neglect the shallows, especially the points off the edges of coves.
There is still much to be learned about the factors that influence foraging behavior in fish while in their natural habitat. When the fish aren’t biting, pondering the internal mechanisms and external factors regulating fish appetite and feeding can be a reasonably entertaining pastime. However, when I’ve hooked into a nice rainbow or grayling on my 4wt fly rod, the science of foraging is honestly the furthest thing from my mind. I want to battle the fish! But I do believe that the knowledge of what factors influence or drive fish to search for food has helped me to become a better angler. It’s also helped me to have a greater appreciation for how complex and interesting these fantastic aquatic creatures truly are.
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Kalb Stevenson is an experienced biologist and fisherman and a long-time Alaskan. He is the owner of Axiom Environmental LLC., a consulting company based in Anchorage, Alaska. Dr. Stevenson has authored numerous peer-reviewed articles, agency reports and popular press pieces in the areas of fish and wildlife ecology and environmental science. He enjoys spending time with family and friends and fishing around the state.
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