Have you heard the term “energy crises?” We are reminded of limits to energy when gas prices jump a quarter per gallon (energy for our vehicles) or grocery prices soar a couple hundred dollars a year above what we spent last year (energy for our bodies).
Although we might not like the price hikes, energy remains available; we just have to pay more to get it.
Fish, on the other hand, truly live in an energy crisis. They need a stable supply of energy (food) to “pay” their metabolic costs. There is no grocery store; fish eat only if they can find food.
Energy consumed above metabolic needs goes to growth, and that includes growing the ovaries that produce eggs and the testes that produce milt.
The effects of abundant or scarce food are obvious to anglers. Fish that live where food is plentiful are chunky for their length and grow fast. Biologists describe the fish as having high condition or high relative weight. The abundant food allows fish easily to meet their metabolic needs and have energy left for growth.
Conversely, when food is scarce fish are thin (have low condition) and grow slowly. Metabolism burns the ingested energy, and none is left for growth or to build reproductive products.
Spawning has a high energy cost. Yes, there are the energy demands for the added activities of building and guarding nests, and courtship. But the far greater energy demand, particularly for females, is the large amount of energy required to develop the reproductive products. For some fish, the eggs, which are high in protein and fats, might be 8 to 14 percent of the fish’s body weight. That’s a lot of growth, and that takes a lot of energy.
The freshwater fish familiar to us in Mississippi are all warm-water fishes that spawn in the spring or summer. Contrary to lore like “fish feed heavily in the spring to store up energy for the spawn,” acquiring the energy to develop the reproductive products does not happen quickly. Further, spring — a time before the forage fish spawn and new crops of invertebrates like crayfish and aquatic insects start their life cycles — is a time when food is at the annual low.
Fish cope with the energetic demands of spawning in one of two ways. Fish that spawn early, like crappies and black bass, must acquire the energy well before the spawn. Ecologists call these fish “capital spawners” — they rely on their stored energy to develop the reproductive products. Capital spawners develop their ovaries and testes in the late summer and fall when food is most abundant, and then they pull energy from their “energy bank account” — energy in their muscle and other living tissues — to finish the process during winter and spring.
The fish are, for the most part, ready to spawn when the temperature climbs into the low 60s for crappie and the high 60s for bass.
Bluegill and other sunfishes spawn later in the spring and in the summer when food is abundant. These fish are able to acquire the necessary energy for reproductive products relatively quickly. Ecologists call these fish “income spawners” — they can predictably acquire enough energy to develop their reproductive products without having to draw on their energy bank accounts.
Whether fish are capital spawners or income spawners, there is a significant consequence to insufficient energy: failure to spawn. The result of too many predators or too little forage is insufficient energy to grow both body tissue and reproductive products.
Populations with low-condition fish might have weak spawns or not spawn at all, while populations with plump fish likely will produce strong year classes annually.
Good management ensures forage supplies are not depleted. And sport fish will have good growth, good body condition, good spawns and produce good fishing.