Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 438

When sampling a fish population, the first questions that arise are usu-
ally how many fish, how big are they, and how old are they? This infor-
mation can indicate several important things about the reproduction,
growth, and mortality of fish and assist with management of populations.
An important consideration is how to catch the fish and determine their
population size (Method 21.1).
After the fish are captured, the total number and sizes of individual fish
must be determined. Length is the easiest to measure but is not related lin-
early to weight (Fig. 21.9B). Because weight is related more closely to vol-
ume than length, weight is a logarithmic function of length for most ani-
mals, including fish. The relationship between length and weight can be
used to assess the condition of fishes. A fish that has a high weight per unit
length often is considered to be healthy and in better condition. Indices
have been constructed to characterize weight-length relationships (Ander-
son and Neumann, 1996). These indices can be used to assess the relative
condition of individual fish within a habitat or to compare the condition
of fish populations among habitats.
As with many animal populations, small fish are usually more numer-
ous than large fish (Fig. 21.9A) because mortality decreases the number of
large fish in a population. Figure 21.9 does not include data on the very
youngest fish because they are not sampled effectively by electrofishing.
The large numbers of fish of about 80 mm in length are probably 1 year
old. An obvious exception to the trend of more small fish than large fish
occurs when adult fish are stocked.
Many indices are used by fisheries managers to quantify growth, mortal-
ity, recruitment, condition, and size structure. I discuss one of these indices that
is based on length data as an example. Relative stock density (RSD) is the per-
centage of fish in a specific length range relative to the total number of fish
above a minimum size estimated to be in the population (Anderson and Neu-
mann, 1996). In other words, it is the proportion of fish in a specific size range
relative to all fish that can be sampled effectively. The RSD offers an index
that describes the distribution of fish lengths in a population. It can be used
in several specific situations. If the index is restricted to just the largest fish in
the population, it can be used to calculate the relative number of trophy fish
in a population. If the index considers all fish large enough for anglers to keep,
it can indicate the quality of the fishery. The RSD can be used to determine if
a fishery is balanced (e.g., that there are not too many small fish or that com-
petition or some other factor is causing poor condition in a specific size class).
The next aspect of population structure I consider is the age of each
size of fish. If the age is known, the rate of growth or the production of the
fish population can be calculated. Two approaches are most often used to
estimate ages of fish—length frequency analysis and analysis of hard parts
of fish (Devrie and Frie, 1996). Length frequency analysis is used where dis-
tinct peaks in the distribution of lengths are observable. Population peaks
occur at specific size classes, each corresponding to a reproductive cohort.
Such peaks may not be discernable when growth and time of reproduction
are variable among individuals and for older fish in the population.
Scales, otoliths, bones, and spines are hard parts that are used to age
fishes. As a fish grows, it adds to these hard structures. Daily marks can
sometimes be discerned, and where growth has a distinct seasonal pattern,