Fernández-Llana, C. (Carmen)

Pérez-Contreras, M.N. (María Nélida)

Trujillo-Gorbea, V. (Valentín)

English

Fernández-Llana, Carmen

Pérez-Contreras, María Nélida

Trujillo-Gorbea, Valentín

Digital.CSIC

Exploring the CPUE of the Vigo-Marín Trawl VII Fleet using Information from Observers on Board

Repositorio Institucional Digital del IEO

EXPLORING THE CPUE OF THE VIGO-MARIN TRAWL VII FLEETUSING INFORMATION FROM OBSERVERS ON BOARDCarmen Ferna´ndez, Ne´lida Pe´rez and Valent´ın TrujilloInstituto Espan˜ol de Oceanograf´ıa, Vigo, SpainApril 27, 20081. Background to the problemThe CPUE of the Vigo-Mar´ın trawl fleet operating in ICES Division VII is used fortuning the XSA-based assessment of the Megrim (Lepidorhombus whiffiagonis) stock inICES Divisions VII b,c,e-k and VIII a, b, d.A series of yearly CPUE (catch per unit effort) is available starting from 1984. Catchconsists of the sum of the estimated landings and discards and effort is measured in fishingdays per 100 HP. What is actually known in any given year is the number of trips (andcorresponding HP) and it is assumed that each trip corresponds to 12.9 fishing days.The following figure shows the historical series of CPUE and LPUE (landings perunit effort) for this fleet. The series that is used for tuning the assessment is the onecorresponding to CPUE, which shows much higher values since 1999:1985 1990 1995 2000 200580100120140160180200Vigo−Marin Trawl VII: CPUE (solid) and LPUE (dashed) −− Kg per Day per 100HPyearkg1The following figure depicts numbers-at-age caught per unit effort for the Vigo-Mar´ınfleet. It is in logarithmic scale and, for each age separately, values have been standardisedover the years (subtracting the mean and dividing by the standard deviation). Clear andblack bubbles represent positive and negative values, respectively. There is an abruptchange in 1999, with most values being negative before that year and positive after it. Itis not possible to track cohorts in this figure, as would have been desirable.Vigo−Marin: For each age separately, standardised log(NCPUE) over the yearsyearage24681990 1995 2000 2005These issues make the use of the Vigo-Mar´ın CPUE series as a relative abundance indexquestionable. In the XSA assessments conducted in the last few years, there is a veryclear pattern in the log-catchability residuals for this series, taking negative values until1998 and positive values since 1999. This indicates that there is disagreement betweenthis source of information and the other sources used in the XSA assessment (commercialcatch matrix and tuning indices from the French FU04 commercial fleet and the UK-WCGFS-D and FR-EVHOES surveys). At the same time, the Vigo-Mar´ın fleet getssubstantial weight in the XSA assessment, which has very strong retrospective patternsthat have led to ICES not having an analytical assessment for this megrim stock since2006.2Table 1: Years with sampling and number of trips sampled1988 1989 1994 1999 2000 2001 2003 2004 2005 2006 20076 5 10 4 7 2 7 8 7 8 92. Analysis of data from observers on boardThe main purpose of this working document is to explore an independent source ofinformation, namely data collected by scientific observers on board, to shed some light onthe issue of whether or not the observed increase in CPUE of the Vigo-Mar´ın fleet mightbe reflective of an increase in stock abundance.The observers data have been collected under the “Spanish ICES Discards SamplingProgramme”, which started in 1988, although it has been discontinued several times.Table 1 shows the years in which sampling has taken place and the number of sampledtrips in each of those years.The following figure displays the total catch (retained plus discarded) for each ofthe sampled trips (open circles), ordered by year. Yearly averages are represented bysolid circles and the horizontal line represents the overall mean level (average of yearlyaverages). Although yearly averages seem to have been a bit higher since 2000, thisincrease is not large and there is clearly a lot of inter-trip variability.31985 1990 1995 2000 20055000100001500020000Catch by trip (open circles)(solid circles are yearly means; horizontal line is mean of yearly means)yearkgThe following figure displays catch by trip per 100 HP (open circles) from the sameobservers dataset. Again yearly means are represented by solid circles and the overallmean level by a horizontal line. The CPUE series that has been used to tune the XSAassessment is overplotted with triangles. The before and after 1999 split seen in the tuningseries is much less clear from the observers data.1985 1990 1995 2000 2005010002000300040005000Catch per 100 HP by trip (open circles)(solid circles are yearly means; horizontal line is mean of yearly means;triangles are WG series)yearkg4As already indicated, in the Vigo-Mar´ın CPUE series used to tune the XSA assessmenteffort each trip is assumed to correspond to 12.9 fishing days. However, the observers dataindicate that the number of fishing hours per trip has been higher in recent years, as thefollowing figure shows:1985 1990 1995 2000 2005100150200250300Trawling hours by trip (open circles)(solid circles are yearly means; horizontal line is mean of yearly means)yearhoursThis increase in number of fishing hours per trip does not seem to be due to a lengtheningof the trips duration but to different working practices on board. The observers dataindicate that in recent years trawling times per haul have been generally shorter withshorter inter-haul time intervals too. The net consequence of this appears to have beenan increase in the total number of trawling hours per trip.The increase in the number of trawling hours per trip suggests that a better measureof effort would be trawling hours or trawling hours per 100 HP. The next two figures showcatch by trawling hour and catch by trawling hour per 100 HP from the observers data:51985 1990 1995 2000 200520406080Catch per trawling hour by trip (open circles)(solid circles are yearly means; horizontal line is mean of yearly means)kg1985 1990 1995 2000 200551015Catch per trawling hour per 100HP by trip (open circles)(solid circles are yearly means; horizontal line is mean of yearly means)yearkgFrom the latter two figures, we see that once trawling hours have been taken into account,the observers data show no indication whatsoever of an increase in CPUE in recent years.So the conclusion from this investigation is that the data from observers on board do notsupport the notion of an increase in the abundance of the megrim stock.63. Possible actions to take regarding the CPUE series of the Vigo-Mar´ın fleetThe fact that neither the other sources of information used in the XSA assessmentnor the data from observers on board analysed in this working document agree with thehigher levels of stock abundance indicated by the CPUE Vigo-Mar´ın series suggests thatthis series is not reflecting stock abundance. Two obvious candidates to be generatingproblems in this CPUE series are:(1) The estimates of discards, which may be too noisy. Sampling to estimate discardshas only taken place in the years indicated in Table 1, so there are many missing years,particularly in the earlier part of the series. Even in the years when there has beensampling, the resulting discards estimates are still very noisy. From the first figure of thisworking document, it is clear that the increase in the index values in recent years wouldless strong if only landings were considered. Hence, a possible course of action would beto remove discards from this series. We note, however, that while eliminating the discardswould attenuate the problem somewhat, it would not remove it. At the same time, sincethe discards estimates are part of the international commercial catch matrix for this stock,it would seem inconsistent not to include discards in the Vigo-Mar´ın tuning series.(2) Effort is not appropriately standardirsed, as it is just computed as number of tripsper 100 HP multiplied by 12.9 days per trip. A proper standardisation of effort of thisfleet is not possible with the data that we have available. A partial solution would be totry and correct the effort in some simple way using yearly mean trawling hours per trip,as estimated from the observers on board data. Two effort correction factors as well as asplit of the tuning series into two parts will be tried. Details follow:“Correction factor 1”: We consider that yearly mean trawling hours per trip were at aconstant level until 1998 and then increased to another level in 1999, as the two horizontalsolid lines in the following figure suggest:71985 1990 1995 2000 2005100150200250300Trawling hours by trip (open circles)solid circles are yearly means; horizontal line is mean of yearly meansleft and right solid horizontal lines are means before and after 1999yearhoursAccording to this idea, the effort of all years until 1998 should be multiplied by 193/265 =0.73 resulting in the following corrected CPUE series:1985 1990 1995 2000 200580100120140160180200Vigo−Marin Trawl VII: CPUE in kg per day per 100 HP (solid)and corrected with trawling hours factor 1 (dashed)yearkg8“Correction factor 2”: Here we assume that yearly mean trawling hours per trip were ata constant level until 1989, then increased linearly from 1998 to 1999, and have remainedat a constant level since then. This is represented by the solid line in the following figure:1985 1990 1995 2000 2005100150200250300Trawling hours by trip (open circles)solid circles are yearly means; horizontal line is mean of yearly meansleft and right solid horizontal lines are means until 1989 and since 1999yearhoursWhen effort before 1999 is corrected according to this idea we obtain the following CPUEseries:1985 1990 1995 2000 200580100120140160180200Vigo−Marin Trawl VII: CPUE in kg per day per 100 HP (solid)and corrected with trawling hours factor 2 (dashed)yearkg9“Two parts split”: A third possible course of action would be to split the tuning seriesinto two, one going until 1998 and another one starting in 1999. This would implicitlysay that effort has changed in the late 1990s in some way that we can not measure ormodel explicitly.In order to explore which of these actions would be more in agreement with the signalcoming from the international commercial catch matrix, we have performed XSA runs(using FLR), tuning only with the Vigo-Mar´ın fleet. Each XSA run was performed usingthe commercial catch numbers-at-age matrix from 1984 to 2006, which goes from ages 1to 10+, no shrinkage, catchability independent of stock size for all ages, q plateau at age8 (the oldest age allowed by XSA) and no taper weighting. Eight options were consideredfor the tuning fleet:1) CPUE of Vigo-Mar´ın2) CPUE of Vigo-Mar´ın with effort correction factor 13) CPUE of Vigo-Mar´ın with effort correction factor 24) CPUE of Vigo-Mar´ın split in two series (no effort correction)5) LPUE of Vigo-Mar´ın6) LPUE of Vigo-Mar´ın with effort correction factor 17) LPUE of Vigo-Mar´ın with effort correction factor 28) LPUE of Vigo-Mar´ın split in two series (no effort correction)Plots of log-catchability residuals for each of these cases follow in the same order:10Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin onlyyearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 2.0 1.5 1.0 0.5 0.0−0.5−1.0−1.5−2.0Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin only −− corrfact1yearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 2.0 1.5 1.0 0.5 0.0−0.5−1.0−1.5−2.011Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin only −− corrfact2yearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 2.0 1.5 1.0 0.5 0.0−0.5−1.0−1.5−2.0Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin only −− two partsyearage2468101985 1990 1995 2000 2005SP−VIGOTR7−84246810SP−VIGOTR7−99Scale 4 3 2 1 0−1−2−3−412Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin Landings onlyyearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 4 3 2 1 0−1−2−3−4Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin Landings only −− corrfact1yearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 4 3 2 1 0−1−2−3−413Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin Landings only −− corrfact2yearage2468101985 1990 1995 2000 2005SP−VIGOTR7Scale 4 3 2 1 0−1−2−3−4Northern Megrim in VII: Log catchability residuals tuning with Vigo−Marin Landings only −− twopartsyearage2468101985 1990 1995 2000 2005SP−VIGOTR7−84246810SP−VIGOTR7−99Scale 4 3 2 1 0−1−2−3−4Note that the range of the residuals is not the same in all 8 plots. Whereas in the first 3plots, residuals go from -2 to 2, in the last 5 plots they go from -4 to 4.14The residuals plots clearly show that there is a discrepancy between the commercial catchmatrix and the Vigo-Mar´ın CPUE tuning series, when effort of this series is not corrected(see first of the 8 residual plots). Applying either of the two proposed correction factors tothe effort of this series has virtually no impact on the residual patterns obtained (secondand third plots). The fourth plot shows a much improved residual pattern (except for age9, when residuals become very large; note the change of range in the bubbles from (-2,2)in earlier plots to (-4,4) in this plot) when the CPUE tuning fleet is split in two parts.The last 4 plots correspond to the LPUE Vigo-Mar´ın tuning series (note that bubbles inthese plots are also in the range (-4,4)). Residual patterns look a bit better than for thefirst 4 plots, showing less of an abrupt change around 1999. Age 2 now has particularlylarge residuals. When the LPUE tuning series is split in two parts, age 9 residuals alsobecome very large (eighth plot).Based on these residual plots we may conclude that the option that is most in accordancewith the signal coming from the international commercial catch matrix is option 4, namelyto split the Vigo-Mar´ın CPUE tuning series into two, with the first one covering from 1984to 1998 and the second one starting in 1999. However, we note that the internationalcommercial catch matrix for this stock contains French discards only until 1998, whichmeans that there has been an artificial decrease in the catch numbers since 1999 in thismatrix. Hence, conflicting signals between the international catch matrix and the Vigo-Mar´ın tuning series, particularly as they relate to a changepoint around 1999, might bein part caused by that problem with the catch matrix.154. Summary of conclusions and final recommendationFrom analysis of the observers on board data (Section 2), we conclude that these do notsupport the notion of an increase in the abundance of the megrim stock in recent years.From XSA runs with 8 possible variations of the Vigo-Mar´ın tuning series (Section 3),we conclude that the option that is most in agreement with the signal coming fromthe international commercial catch matrix for this stock is the one corresponding to asplit of the Vigo-Mar´ın CPUE series in two parts (until 1998 and since 1999). We note,nonetheless, that the fact that the catch matrix contains French discards only until 1998may be causing part of the discrepancies seen between this matrix and the Vigo-Mar´ınCPUE series.Basing ourselves mainly on the analysis from the observers on board data and to a lesserextent on the comparability with the international commercial catch matrix (notwith-standing the problems with it just mentioned), we suggest that the Vigo-Mar´ın CPUEtuning series be split into two, with the first one covering from 1984 to 1998 and thesecond one starting from 1999.16

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Exploring the CPUE of the Vigo-Marín Trawl VII Fleet using Information from Observers on Board

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