Here Comes Winter 

Fish Need Energy, Oxygen, and Not be Eaten by Predators 

By Eric West 

With winter near and around us it is time to look at some elements of fisheries management that occur when the water is cold. Pond fish species are cold blooded and that means their metabolism and activities slow with the drop into winter temperatures. There are, however, still important things going on under ice or winter cold water in your fishery.  

Fish still have to eat and breathe.  

Early on, scientists examined how winter temperatures impacted fish, especially in the areas of winter survival which depends on food consumption (or fat reserves) and predation. As early as 1945 James W. Moffett and Burton P. Hunt in Winter Feeding Habits of Bluegills, Lepomis Macrochirus Rafinesque, and Yellow Perch, Perca flavescens (Mitchell), in Cedar Lake, Washtenaw Michigan published in Transactions of the American Fisheries Society, 73:1, 231-242 (1945) examined the interaction between bluegill and yellow perch. They found that bluegill, a warm water species, consumed very little food during winter. The average stomach contained small amounts of various planktons. Their diet was predominantly aquatic insects (mayfly nymphs) in early winter until they were no longer available, which changed to planktons (mostly Cladocera) in midwinter and tended again toward aquatic insects as they reappeared with initial spring warming. One interesting note for pond owners who feed pellets was the correlation noted between winter warm spells and an increased rate of food consumption for bluegill. Many on the Pond Boss Forum have reported the same and reveal that they feed in limited amounts during winter’s warm snaps often using hydrated pellets. A little extra food during the cold of winter can go a long way toward increasing survival and condition. This study also revealed that yellow perch (a cool water fish species), were more active and ate much more food than did bluegill during winter. Amazingly, those cool water yellow perch were eating those small, cold, slow-moving bluegill, which accounted for 60% of their diet. During some warm periods, the yellow perch consumed some seed shrimp (Ostracoda) in great numbers. An important management point for our northern readers is the authors observation that winter predation by perch on young bluegills certainly was an important factor in regulating the bluegill population and aiding yellow perch condition. 

Some 53 cold winter years later Daniel E. Shoup and David H. Wahl examined largemouth bass and bluegill winter survival, feeding and growth in The Effect of Largemouth Bass Predation on Overwinter Survival of Two Size-Classes of Age-0 Bluegills in Transactions of the American Fisheries Society, 137:4, 1063-1071 (2008). This study examined winter predation on two sizes of small bluegill by two sizes of largemouth bass in small Illinois ponds. This study was the first quantification of predation effects on overwinter survival of young bluegill by largemouth bass. Predation accounted for a 16-49% increase in mortality above that occurring in predator-free ponds, confirming that predation can cause significant overwinter mortality of bluegill populations. In northern areas this predation by largemouth bass on bluegill further helps in both bluegill condition (less mouths to feed) and in reduction of possible bluegill stunting for the same reason. 

A key to fisheries population management is the concept of total mortalities. We all know that many more fish are hatched than can be supported by the pond. Very high percentages must be eaten or die or be harvested in order to keep the rest healthy. For many species, like bluegill, yellow perch and Largemouth bass, first-year overwinter mortality is critical to year-class size and condition. Reduced survival during winter has been attributed to low food availability (starvation), metabolic inefficiencies, reduced swimming ability (lipid imbalance due to cold), osmotic stress, and increased vulnerability to disease, predation, and low oxygen availability. 

Dissolved oxygen is critical to survival and in ice covered northern ponds especially. One study titled The Influence of Dissolved Oxygen on Winter Habitat Selection by Largemouth Bass: An Integration of Field Biotelemetry Studies and Laboratory Experiments by C. T. Hasler, C. D. Suski, K. C. Hanson, S. J. Cooke, and B. L. Tufts in Physiological and Biochemical Zoology 82.2 (2009): 143-152 looks at this issue. The biotelemetry study compared dissolved oxygen levels measured throughout the winter period with continually tracked locations of adult largemouth bass. Habitat usage was compared with habitat availability to assess whether fish were keying on specific dissolved oxygen levels. Results from the dissolved oxygen measurements made during the biotelemetry study showed high variance in under‐ice dissolved oxygen levels. Avoidance of water with dissolved oxygen under 2.0 mg/L was demonstrated, but significant use of water with intermediate dissolved oxygen levels was also found. The laboratory study examined behavioral and physiological responses to winter temperatures in juvenile largemouth bass. Results from the lab experiments showed marked changes in behavior with low dissolved oxygen. Results of the biotelemetry and laboratory studies demonstrate that adequate dissolved oxygen content is critical during the winter. Further, the use by fish of areas with intermediate levels of dissolved oxygen suggests that there are multiple environmental factors influencing winter behavior. Overwinter mortality of young fish is often size specific, affecting smaller individuals to a greater extent than larger individuals presumably because large individuals have greater energy reserves and lower mass to metabolic rates. 

Studies of bluegill and other fishes indicate size-specific interactions are important during winter. Winter food availability is often low, making the buildup of energy reserves before winter important. However, chasing down and accumulating those reserves often comes at the cost of reduced growth and an increased risk of size specific predation. Size-specific predation risk effects a host of other interactions, including changes in habitat and diet and ultimately competition among both different size-classes and species for food resources. Year class size and mortality rates are very complex and result in outcomes that are exhibited differently in different waters. Studies like these show complex interactions, but critical factors for winter survival are abundant energy reserves, enough size to reduce predation risk and at least enough oxygen to breathe. 

Studies show that the colder the climate, for most species, the higher the winter mortality on young fish. For example, many more fish die due to cold induced starvation and low oxygen in northern ponds than in warm southern ponds. However, mortalities may increase in southern ponds because activity levels remain high, food consumption/depletion is high and predation continues. Either way there are increased winter mortalities of small fish.  

Understanding the basics of fish biology including the food web (chain) and its impacts on fish through the seasons is important. While we are out a lot more in spring, summer and fall observing and understanding, don’t discount the impact of cold-water winter. Pond life is after all a giant puzzle. The more you understand the seasonal effect th, e easier it is to work the puzzle.