Fish Population Dynamic and Stock Assessment

9.2.4 Standardisation of fishing effort

In tropical fisheries, different gears are used to capture the same resources. For instance, the sharks are captured by gillnets, trawls, hook and lines, etc. The catching efficiency between these gears differs widely. It is also difficult to compare effort of different vessels operating a single gear. For example, horsepower of trawler engines ranges from 80 to 150 , and hence, fishing hours multiplied by horsepower may a suitable measure of effort in trawl fisheries. In a gillnet fishery, the engine horsepower and fishing hours are less important compared to the number of gillnets set per day. In a hook and line fishery, it may be appropriate to consider the number of fishermen multiplied by the number of hooks used.

An attempt to combine the effort of these gears encounters intricate problems. Sparre and Venema (1992) suggested a method in which the quantities of yield and CPUE are proportional to effort.

(i) Relative effort

$${yield \over CPUE} = Effort as CPUE = Yield / Effort$$

(ii) Relative CPUE

The effort of a particular gear (gear i) in year (y) is defined as relative catch per unit effort

Ri (y) = $${CPUE i(Y)\over CPUE i(Y1,Y2,.....)}$$………………….. (1)

Relative CPUE is calculated as follows,

Add CPUE for different years

9 + 5.7 + 4 + 3.9 + 3 = 25.6 (cumulative CPUE for five years)

Divide the cumulative CPUE with the number years of observation (say five years)

$${25.6\over 5}$$= 5.12, Using the equation 1 the relative CPUE for the years in observation could be calculated.

In reality some gears are less important than others. For example, if two gears namely Purse seine, Beach seine and Pole and line and Trolling line are taken into account, the relative CPUE is calculated as follows.

Y	Yield y/1 (y)	f (y)	CPUE1- (y)	Ri (y)		Yield (y) y/f	f (Y)	CPUE2 (Y) (y/f)	Ri (y)
2005	50		10	1.02		250	20	12.5	0.94
2006	200	20	10	1.02		200	15	13.3	1.002
2007	400	43	9.30	0.95		350	25	14.0	1.055

Mean CPUE for purse seine

$${10+10+9.30\over 3}$$=$${29.3\over 3}$$ = 9.78

Mean CPUE for beach seine

$${12.5+13.3+14\over 3}$$=$${39.8\over 3}$$ = 13.2

Relative CPUE

R (1980) = $${50 x 1.02+250x0.94\over 50+250}$$= 0.95

Similarly calculate relative CPUE for both the gears for the year of observation.

Assuming YT (y) (Total yield of all gear – including gears for which effort is not known) is taken as 3000 for the year 2005, 4000 for the year 2006 and 7500 for the year 2007, the relative effort of year YT (y) / R(y) for 2005 is calculated as 3000 / 0.95 = 3158 and for the year 2006 the relative effort is 4000 / 1.011 = 3957.

The normalized relatively effort E(y) for the particular year is calculated by

E(y) = $${YT(y)/R(y) \over mean YT/R}$$