VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

ESTIMATING GROWTH BY LENGTH-FREQUENCY ANALYSIS

• The length frequency (LFD) analysis can be carried out following peterson’s method

Method

• Remove otoliths by cutting across the dorsal sides of the head with a sharp scalpel behind the eyes.
• Remove the otolith from the otic capsule, wash in water and weigh each otolith to the nearest 0.01 mm and 0.001 mg, respectively.
• Grind the otoliths, immerse in 50% glycerol and observe under binocular microscope to see the annual rings.
• The growth parameters can be estimated using the Von Bertalanffy growth equation (VBGE), which can be written as:

Lt=L∞(1-e ^–k(Lt)0)

Where Lt = length at age t, L∞ (= asymptotic length (the length of the fish would reach at an infinity age), K= the growth coefficient (constant), t= age of the fish and t₀ is the theoretical age the fish would have at length zero. The condition or well being of fish can be determined by weight relationship (LWR) using the parabolic fromula and its logarithmic transformation, respectively.

W=a+L^b (Le Cren, 1951)

And

Log W = Log a + b log L

where W is the weight of fish (g), L denotes the length of fish (mm), ‘a’ is the intercept and ‘b’ is the slope, ‘a’ and ‘b’ are constants.

Direct observation of growth

• Growth can be observed directly in fish raised in captivity, where length progression can be followed throughout the rearing period. Growth in captivity is not, however, comparable to growth under natural circumstances.
• Growth can be established in sedentary populations which can be observed in the same place over long periods of time. This information is obtained by photographs or by drugging the fish and measuring them directly.