Chains, Webs And Carrying Capacity

 

With nearly $4-per-gallon gas being a reality for most of us, we all know how energy needs affect our budget and lifestyle. Fish, too, face an energy crisis. Indeed, they live in a perpetual one.

 

Aquatic ecology 101: In the underwater world, energy from the sun is “fixed” (that is, converted from solar energy to organic compounds like sugars and starches) primarily by algae suspended in the water column (phytoplankton). Ecologists call these energy fixers “primary producers.”

Phytoplankton is consumed by zooplankton—microscopic animals suspended in the water—and by a variety of other invertebrates like insect larvae. Zooplankton are then eaten by larger invertebrates, the young of all species of fish, and some adult fish, like minnows and shad. The larger invertebrates are important foods for fish like minnows, suckers, sunfish and perch. These fish are, in turn, eaten by sportfish. 

The energy fixed by primary producers passes through food chains—or more complex pathways called food webs—and ultimately buildssportfish populations. The amount of energy passing through the food chains and webs determines the weight or biomass at each link in the chain.

There is a catch, though. Only about 10 percent of the energy at one level passes to the next level, so 10 percent of the phytoplankton is converted to zooplankton, 10 percent of that energy is converted to invertebrates or forage fish, and so on.

 

The biomass at each link that can be sustained by the energy from primary producers is called the carrying capacity. Many of the most highly valued sportfish feed on other fish, so a lot of energy is lost as the energy passes through the food web, and the carrying capacity for these top predators is relatively low.

Fish, like all living organisms, need energy to stay alive. After basic metabolic needs are met, additional energy is available for growth and reproduction.

 

What does this fundamental ecology have to do with fishing? Almost everything, because growing fish populations is about energy. Think about how far-reaching the following three points are:

 

1. Primary production is limited by nutrients like nitrogen and phosphorus. Nutrient-rich (eutrophic) lakes have more primary production (and greener water) which supports more fish biomass. Largemouth bass biomass will top out at 40 to 50 pounds per acre in eutrophic lakes. Lakes with lower nutrients have less phytoplankton (and clearer water) but support less fish biomass. Mid-fertility (mesotrophic) lakes may support only 5 to 10 pounds of walleyes per acre.

 

2. When a fish population is at carrying capacity, energy becomes limited and growth slows. Reducing harvest with high length or low creel limits will only grow larger fish if the population is below carrying capacity. A corollary to this is that fish need to be harvested when a population has high recruitment and slow growth if the intent is to provide anglers with fast-growing, large fish.

 

3. Stocking accomplishes little or nothing when a reproducing population is at carrying capacity. The added fish compete for energy with the existing fish, growth slows and natural mortality increases. When a population does not have natural reproduction, which may be the case in some walleye, muskie or striped bass populations, alternate-year stocking often is more successful.

Not all lakes are equal—they support different species and have different fish biomass. All fish populations, though, have a carrying capacity controlled by energy, and energy—and the management of energy flow—is the foundation of good fishing.