Subjective judgement in data subsetting: implications for CPUE standardisation and stock assessment of non-target chondrichthyans

Abstract

Abstract. Standardisation of catch-per-effort (CPUE) data is an essential component for nearly all stock assessments. The first step in CPUE standardisation is to separate the comparable from the non-comparable catch and effort records and this is normally done based on subjective rules. In the present study, we used catch-and-effort data from the elephant fish (Callorhinchus milii) to illustrate the differences in CPUE when using expert judgement to define different ad hoc selection criteria used to subset these data. The data subsets were then used in the standardisation of CPUE and the stock assessment of elephant fish. The catch-and-effort subsets produced different patterns of precision and trends, each of which led to different estimates (and related uncertainty) of model parameters and management reference points. For most CPUE series, there was a very high probability that the elephant fish stock is overexploited and that overfishing is occurring. The estimates of total allowable catch (TAC) and the uncertainty around these estimates also varied considerably depending on the CPUE series used. Our study shows how sensitive TAC estimation is when there is high uncertainty in the definition of the fishing effort targeted at the species analysed.

Standardized catch rate of cockfish, Callorhinchus callorynchus, in a bottom trawl fishery of Patagonia: Is it possible its use as a predictor of abundance trend?

“…This model registered the lowest AIC value, presented a good fit of the observed data and explained a 23.5% of the total variability (Table 2). This model selection agrees with other studies of chondrichthyans that are captured as bycatch which used the same error distribution for the positive data set (AIRES et al, 2008;MINISTRY FOR PRIMARY INDUSTRIES OF NEW ZEALAND, 2011;BRACCINI et al, 2011;BARNETT et al 2012;CARLSON et al, 2012). Generally, the models used to standardize the CPUE explained a low percentage of the total variability of the data set.…”

“…However, the most common method is the use of Generalized Linear Models (GLM) (MAUNDER and PUNT, 2004) and recent applications for sharks have included extensions of Generalized Linear Mixed Models (GLMM) (BAUM and BLANCHARD, 2010;BRACCINI et al, 2011). These models allow to estimate the catch rate (response variable) with a linear combination of a set of explanatory variables.…”

“…In 1986, the C. milii stocks of New Zealand were declared overexploited, but then they showed recovery signs up to reach historic landing volumes, reaching the maximum landings of 1,300-1,400 tons approximately between 2007(FRANCIS, 1998MINISTRY FOR PRIMARY INDUSTRIES OF NEW ZEALAND, 2011). In Australia, BRACCINI et al (2011) evaluated the use of the standardized CPUE as an index of relative abundance for C. milii, and unlike the New Zealand case, they detected a decreasing tendency during the study period from 1976 to 2006.…”

“…Particularly, the species of the genus Callorhinchus are captured and commercially exploited in all their distribution range in the south hemisphere: the elephant fish C. milii in New Zealand (FRANCIS, 1998) and Australia (BRACCINI et al, 2011), the St Joseph C. capensis in South Africa (FREER and GRIFFITHS, 1993) and the cockfish C. callorynchus in Argentina (DI GIÁCOMO and PERIER, 1991) and Chile (ALARCÓN et al, 2011).…”

“…Therefore, standardization of CPUE considering the effect of these factors and the particular dynamic of each fishery and each species (PUNT et al, 2000;MAUNDER et al, 2006;BRACCINI et al, 2011) is necessary to construct reliable abundance indices (HILBORN andWALTERS, 1992, MAUNDER and. The studies about the abundance trends of chondrichthyans that are caught as bycatch and the factors that influence their catch rate have been increased worldwide both in multispecies and monospecies fisheries (PUNT et al 2000;SIMPFENDORFER et al, 2002;BAUM et al, 2003;AIRES da SILVA et al, 2008;BAUM and BLANCHARD, 2010;CARVALHO et al, 2010;BRACCINI et al, 2011;BARNETT et al, 2012;CARLSON et al, 2012;BROMHEAD et al, 2012, TRAVASSSOS TOLLOTTI et al, 2013.…”