How Many Fish Need to Be Measured to Effectively Evaluate Trawl Selectivity?

RESEARCH ARTICLE How Many Fish Need to Be Measured to Effectively Evaluate Trawl Selectivity? Bent Herrmann1,5☯, Manu Sistiaga2☯*, Juan Santos3, Antonello Sala4 1 SINTEF Fisheries and Aquaculture, Fishing Gear Technology, Willemoesvej 2, 9850, Hirtshals, Denmark, 2 SINTEF Fisheries and Aquaculture (SFA), Brattørkaia 17C, N-7010, Trondheim, Norway, 3 Thünen Institute for Baltic Sea Fisheries, Alter Hafen Süd 2, Rostock, 18069, Germany, 4 National Research Council (CNR) – Institute of Marine Sciences (ISMAR), Ancona, Largo Fiera della Pesca – 60125, Ancona, Italy, 5 Norwegian College of Fishery and Aquatic Science, University of Tromsø, 9037 Breivika, Tromsø, Norway ☯ These authors contributed equally to this work. * a11111 OPEN ACCESS Citation: Herrmann B, Sistiaga M, Santos J, Sala A (2016) How Many Fish Need to Be Measured to Effectively Evaluate Trawl Selectivity? PLoS ONE 11 (8): e0161512. doi:10.1371/journal.pone.0161512 Editor: Andrea Belgrano, Sveriges lantbruksuniversitet, SWEDEN Received: January 18, 2016 Accepted: August 5, 2016 Published: August 25, 2016 Copyright: © 2016 Herrmann et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract The aim of this study was to provide practitioners working with trawl selectivity with general and easily understandable guidelines regarding the fish sampling effort necessary during sea trials. In particular, we focused on how many fish would need to be caught and length measured in a trawl haul in order to assess the selectivity parameters of the trawl at a designated uncertainty level. We also investigated the dependency of this uncertainty level on the experimental method used to collect data and on the potential effects of factors such as the size structure in the catch relative to the size selection of the gear. We based this study on simulated data created from two different fisheries: the Barents Sea cod (Gadus morhua) trawl fishery and the Mediterranean Sea multispecies trawl fishery represented by red mullet (Mullus barbatus). We used these two completely different fisheries to obtain results that can be used as general guidelines for other fisheries. We found that the uncertainty in the selection parameters decreased with increasing number of fish measured and that this relationship could be described by a power model. The sampling effort needed to achieve a specific uncertainty level for the selection parameters was always lower for the covered codend method compared to the paired-gear method. In many cases, the number of fish that would need to be measured to maintain a specific uncertainty level was around 10 times higher for the paired-gear method than for the covered codend method. The trends observed for the effect of sampling effort in the two fishery cases investigated were similar; therefore the guidelines presented herein should be applicable to other fisheries. Data Availability Statement: All relevant data are within the paper and the appendix. Funding: The following institutes funded this work: SINTEF Fisheries and Aquaculture (Norway)(www. sintef.no), National Research Council (CNR) – Institute of Marine Sciences (ISMAR) (Italy) (www. ismar.cnr.it) and the Thünen Institute for Baltic Sea Fisheries (Germany) (www.ti.bund.de). Competing Interests: The authors have declared that no competing interests exist. Introduction The development of the selective properties of fishing gears towards desired species or multispecies specific size selectivity is a widely used approach to attempt to achieve more sustainable fisheries [1]. Therefore determining the size selective properties of fishing gears is important for fisheries management. Mostly, the estimation and optimization of the size selective PLOS ONE | DOI:10.1371/journal.pone.0161512 August 25, 2016 1 / 22 Number of Fish Measured and Uncertainty in Trawl Selectivity properties of fishing gears has been carried out based on analyzing data collected from experimental fishing. This is the case also for trawls, which represent one of the most important fishing methods used worldwide. The most basic measure of the size selective performance of a trawl is quantification of the size selectivity of the different species captured by the gear in individual hauls. The most common procedures applied to assess size selectivity in trawls and other towed fishing gears are outlined in cooperative research report no. 215 titled "Manual of methods of measuring the selectivity of towed fishing gears" [2]. The assessment of size selectivity in trawls has traditionally focused on the codend, and the majority of scientific studies conducted on codend size selectivity apply a size selection model in which the probability that a fish will be retained by the gear increases with increasing fish size. The model most often applied in these studies is the logit model [2]. Numerous examples of the application of the logit size selection model to describe size selectivity in trawl codends can be found in the literature (e.g., [3–14]). The quantity and diversity of studies that have used this model demonstrate its relevance and suitability for size selectivity research. As described in Wileman et al. [2], the logit size selection model for a single haul can be fully described by two parameters, L50 (the length of fish with 50% probability of being retained by the gear) and SR (difference in length of fish with respectively 75% and 25% probability of being retained by the gear). Thus, many size selectivity studies for trawls include the assessment of L50 and SR for individual hauls. In scientific studies, an important aspect of assessing the value of model parameters such as L50 and SR is the assessment of the uncertainty in the values, often quantified by the 95% confidence intervals (CI's). Without the CI's, the parameter estimates themselves have little value as the CI's define the limits for the advice fishing gear scientists and fisheries managers can provide based on the analyzed size selection data. The assessment of the uncertainty in L50 and SR at haul level is also of essential importance even when this assessment is only a middle step to finally estimate the mean size selection for a group of hauls. The model of Fryer [15] for example, which has been widely used in selectivity studies, requires an initial analysis step in which the selection parameter values and uncertainties for each haul need to be estimated in terms of the parameters' covariance matrix [2, 15]. Hence, the estimation of the uncertainty of L50 and SR in individual hauls based on a logit selection model can be considered as an important aspect for the majority of trawl size selectivity studies. This leads to a number of questions related to how many fish would need to be caught and measured in a typical t (...truncated)