What is the catch with self-sampling? Comparing catch and discard estimates from co-sampled trips

Sven Sebastian Uhlmann, A.T.M. van Helmond, Aukje Coers, Rosemarie Nijman, Ronald Bol

    Onderzoeksoutput: Bijdrage aan congresPaper

    Uittreksel

    When fishers collect specimens for research purposes (‘self sampling’), they potentially non-randomly pick and/or miss a species or their sizes and thereby bias the subsample for subsequent biological analysis. Likewise, scientific observers may also routinely not adhere to the sampling protocol. Here we compare two different methods of sampling discards: observer versus self-sampling. To establish deviation between observer- and self-sampled estimates of total catch volumes, discard per unit effort rates, and length-frequency distributions, we compared data from 24 co-sampled hauls during 12 commercial fishing trips between mid-2011 and 2012 in The Netherlands. During co-sampling of a trawl gear, an observer and fisher each retain from the haul independently 40 or 80 kg of discards, respectively. However, to scale this to the total volume of discards per haul requires the accurate estimation of the total catch volume which can be biased depending on catch composition, environmental conditions, vessel construction and level of experience. On five trips, observers either significantly over- (n=3 trips) or underestimated (2) total catch volumes compared with fisher’s estimates, but on average across all trips the deviation was only 5%. We observed no significant differences in discards rates and length distributions for dab, whiting, sole and plaice. Under- or overestimating biases between observer and self-sampling seem to even out, at least for the metrics considered here. However, whether the integration of self-sampled discard data into stock assessments is justifiable, will depend on the deviation of vessel-specific estimates between the trips that are co-sampled and those that are not.
    Oorspronkelijke taalEngels
    PublicatiestatusGepubliceerd - 2014
    EvenementFishery Dependent Information Conference - Rome, Italië
    Duur: 3-mrt.-20147-mrt.-2014

    Congres

    CongresFishery Dependent Information Conference
    Land/RegioItalië
    StadRome
    Periode3/03/147/03/14

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