Ratio Estimation Method to Estimate a Bycatch Rate

The ratio method has been used to estimate harbor porpoise and seal bycatch in the US North Atlantic gillnet fisheries.  Landings were considered the most appropriate unit of effort for this fishery because it is presently the only unit of effort that is accurately recorded and available for the entire fishery.
 
The total annual number of marine mammal takes (B) is defined as the product of the observed annual bycatch rate and total annual effort within each stratum (S):

ratio

Strata are spatially and temporally specific, taking into account the seasonal distribution of the bycaught species and gillnet fishing practices. 

A weighted bycatch rate was calculated for strata within the Northeast gillnet fishery that had hauls fishing with and without pingers, where the weight was the proportion of hauls sampled with or without pingers.

 

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(File Modified Jan. 11 2018)