Chapter 15 Length, Weight, and Associated Structural Indices

Common measurements of fish length: maximum standard, fork, and maximum total.

Typical pattern of effects on length and weight that might be observed for fishes preserved in 10% formalin.

Weight-length relations (A and B) and relative weight (

Wr

) plotted as a function of length (C) for largemouth bass collected from Murdo Lake, South Dakota. Graph A: the curvilinear relationship between weight and length (

W = aL b ).

Graph B: The weight-length relationship for the same data that were transformed using common logarithms [log 10

W = a' + b

( log 10

L

)]. Graph C: mean

Wr

per centimeter length group for 20 cm and longer largemouth bass collected by spring electrofishing [(

Wr

data from Lindgren 1991)]

Standard Weight (Ws) Equation

log 10 Ws = - a + b log 10 total length

Relative Weight (Wr) Equation

Wr = (W/Ws) x 100

Wr = 120

Wr = 100

Wr = 80

15.5

Relationship between mean

Wr

of largemouth bass 8-11.9 in long and biomass of prey from 18 Midwestern ponds (from Wege and Anderson 1978)

Relationship between estimated growth increment for 6 months and

Wr

for age 2 largemouth bass 9.8-10.2 in long at time of capture in 18 Midwestern ponds (from Wege and Anderson 1978).

Trends in mean

Wr

across total length for three lake samples of walleye (from Murphy et al. 1990). Note: calculation of mean

Wr

for the entire sample in each lake would mask important length-related trends in body condition.

Live-weight growth efficiency of bluegills fed midge larvae as a function of relative weight at the end of the experiment [Data from experiment D by McComish (1971)].

Logarithmic plot of cumulative available prey/predator (

AP/P

) for three general conditions: (A) prey excesses for all lengths of predators; (B) prey deficiencies for all lengths of predators; and (C) adequacy for small predators but excess for large predators (>20 cm). Diagonal line indicates minimum desirable AP/P ratio (from Noble 1981).

Length-frequency distributions of age-1 and older bluegills (> 5 cm) sampled from angler catch, electrofishing, and lake draining at Ridge Lake, Illinois, fall 1985 [from Santucci and Wahl (1991)]. Estimated proportional stock density (PSD) values for these three sampling methods were 63 for Angling, 16 for electrofishing, and 18 for lake draining.

Proportional Stock Density (PSD)

PSD



Number of Number of fish fish

 

quality length stock length x

100

Relative Stock Density (RSD)

RSD



Number of Number fish of



specified length fish



stock length x

100

Stock Density Index Ranges for Balanced Populations Species Bluegill Crappie Largemouth bass Northern pike Walleye Yellow perch PSD 20-60 30-60 40-70 30-60 30-60 30-60 RSD-P 5-20 >10 10-40 RSD-M 0-10 0-10

Weithman's relationships used to calculate fish quality that was adopted to identify length ranges from or near which minimum stock, quality, preferred, memorable, and trophy lengths were selected for various species (from Gabelhouse 1984).

Relationship between proportional stock density (PSD) and catch per hour of electrofishing (CPUE) for largemouth bass 20 cm and longer in 13 small South Dakota impoundments (from Guy and Willis 1990)

Relationship between relative stock density of preferred length largemouth bass (RSD-P) and catch per hour of electrofishing (CPUE) for largemouth bass 20 cm and longer in 13 small South Dakota impoundments (from Guy and Willis 1990).

Relationship between bluegill mean backcalculated length at age 6 and catch per hour of electrofishing (CPUE) for largemouth bass 20 cm and longer in 9 small South Dakota impoundments (from Guy and Willis 1990)

Relationship between bluegill mean backcalculated length at age 5 and relative stock density of preferred length largemouth bass (RSD-P) in 10 South Dakota impoundments (from Guy and Willis 1990).

Relationship between bluegill proportional stock density (PSD) and catch per hour of electrofishing (CPUE) for largemouth bass 20 cm and longer in 10 small South Dakota impoundments (from Guy and Willis 1990).

Relationship between largemouth bass PSD and bluegill PSD in Midwestern ponds studied by the Central States Pond Management Work Group (from Anderson 1978).

Relationship between electrofishing catch rate of quality (> 6 in) bluegills and stock length (> 8 in) largemouth bass from 23 Kansas state fishing lakes (from Gabelhouse 1984)

Relation between proportional stock density (PSD) and density (number/ha) and between PSD and biomass (kg/ha) of brook trout in Wyoming beaver ponds (top; Johnson et al. 1992) and largemouth bass in small South Dakota impoundments (bottom, Hill and Willis 1993).

Phase-plane graph showing changes in mean relative weight (

Wr

) and proportional stock density (PSD) of largemouth bass collected from 1989 to 1993 in Murdo Lake, South Dakota after implementation of a 300-380 mm slot length limit in 1989 [from Neumann et al. (1994)].

Phase-plane graph showing the time trajectory of weighted proportional stock density (PSD) for game fish (largemouth bass and northern pike) and panfish (bluegill, yellow perch, and pumpkinseed for two Wisconsin lakes. Mid Lake was opened to angling in 1976 after having been closed for 20 years. Allen Lake had been continuously exploited [from Goedde and Coble 1981)].