Diel variation in the deep-water rose shrimp Parapenaeus longirostris (Lucas, 1846) catches off the Portuguese south coast
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1
IPMA-Instituto Português do Mar e da Atmosfera, Portugal
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2
CCMAR-Centro de Ciências do Mar Universidade do Algarve, Portugal
INTRODUCTION. Research surveys are key-elements in supporting the assessment of marine resources, namely by estimating fisheries-independent abundance indices. As such, guaranteeing the accuracy of estimates, by avoiding potential bias in data collection, should be a primary concern. Amongst bias-inducing factors, changes in catchability - the relationship between fish abundance and actual capture - is one of the most problematic, being often assumed either as constant or estimated as a model output instead of experimentally derived.
Several factors such as cyclic burrowing or migratory behaviour may determine the catch process of a given species, affecting the catchability, as occurs in benthic, nektobenthic or pelagic crustacean species. A former study on the ecology and fishery of deep-water rose shrimp Parapenaeus longirostris (Lucas 1846), hereinafter rose shrimp, carried out in the south of Portugal, first reported higher Catch Per Unit Effort (CPUE) during the day compared to night hauls.
This study aimed at the characterization of the diel variability in the rose shrimp catches, and understanding its potential drivers. Recommendations and advice are given concerning the potential impact of changing catchability in research surveys addressing the estimation of stock abundance.
METHODS. Rose shrimp catch data were collected during an experimental bottom trawl survey carried out in September 2007 off the south coast of Portugal aboard the R/V Noruega, a 1500 HP stern trawler belonging to IPMA. The experimental gear consisted of a modification of the crustacean sampling trawl where a horizontal netting panel was placed between the upper and lower trawl bellies, dividing the net into two equal and independent longitudinal compartments connected to two different codends. This setup aimed at evaluating the species distribution in the water column, simultaneously with diel differences in CPUE.
Fishing was carried out over five 24-hour cycles at depths ranging between 238 and 251 m, resulting in a total of 20 hauls in rose shrimp fishing grounds between longitude 08º08’ - 07º04’W and latitude 36º07’- 36º03’ N.
As all hauls had an effective fishing duration of 1-hour, adopted as the unit fishing effort, catches are expressed as the number (or weight) of fish caught per hour (CPUE). The overall catch weight was registered and the most important crustacean and fish species measured and sexed. The Mann-Whitney test was used to compare catches between day and night-times, and between codends. The χ2-test was used to check if the overall sex ratio was significantly different from 1:1 and for differences over the diel cycle. To investigate the effects of codend, diel cycle, sex, and their interactions, in the rose shrimp catches, an analysis of variance (ANOVA) was carried out.
The catch of rose shrimp CPUE in both codends (in number) was modelled as a response variable by Generalized Additive Model (GAM) assuming a Poisson distribution via log-link function, with time of the day (solar hour) as the predictor variable.
RESULTS. The daytime CPUE in weight for rose shrimp of 16.9 Kg/h±7.9, was significantly greater (24 times; p<0.0001) than the night-time CPUE of 0.7 Kg/h±0.7. Differences in number (1479±827 individuals collected during the day versus 79±76 by night) were also significant (p<0.001).
Differences between codend catches within the diel cycle were also tested, resulting in significant daytime differences (p<0.05), with the lower codend presenting a higher average CPUE (10.89 Kg/h) than the upper codend (6.02 Kg/h). Although not significant, the same trend was registered in CPUE in number: 976 N/h in the lower codend and 520 N/h in the upper one. During night-time, catches were higher in the upper codend both in biomass (0.49 Kg/h) and in numbers (42 N/h), compared with lower codend; 0.24 Kg/h and 37 N/h, but not significant (p>0.05).
Rose shrimp females were consistently larger (t-test; p<0.001) and more abundant than males in all scenarios analysed, independently of the time of the day, representing an average proportion of 0.55 of the total catch (χ2 test Ho=0.5; p<0.001), an average of 0.58 of the day catch (p<0.001) and an average of 0.70 of the night catch (p<0.001). ANOVA highlighted the codend (p<0.001), diel cycle (p<0.001) and the interaction of the two (p<0.01) as the factors that most influenced rose shrimp catches.
GAM resulted in an adjusted R2 of 0.45, explaining 73.8% of the total model deviance. Rose shrimp CPUE showed a sharp increase at dawn (between 6:00 and 10:00h), attaining a maximum before noon with some variation thereafter, followed by a rapid decline at dusk (from about 17:00h; Figure 1).
CONCLUSIONS. There is a striking difference between day and night catches obtained in the commercially exploited rose shrimp fishing grounds, confirming that similarly to other nektobenthic shrimps, this species displays a diel migration from daytime to night-time grounds. During daytime shrimps stay on or relatively close to the bottom, forming loose aggregations (‘swarms’), being mainly retained in the lower codend. At night, the majority of the population migrates while remaining individuals tend to disperse, being retained in the upper codend.
Migration is seemingly carried out to lower depths and is triggered by trophic reasons. In fact, a survey carried out in Mediterranean waters, at 140-160 m, revealed higher catches during the night. Also, echograms obtained throughout the current experiment show a dense scattering layer (SL) at night, where rose shrimp also feed on, rising from the bottom to about 100 m. Although SL may rise almost to the surface, in the upper shelf it seems to remain close to the bottom. At dawn, as light increases and the SL migrates downwards to its daytime depths, rose shrimp follow the opposite path. It is conceivable that, as referred in the literature for similar cases, the return to the original daytime depth range is driven, not only by feeding but also to avoid predation.
Diel variability in rose shrimp catch, as observed in this study, may have a major impact on the catchability of survey gears and hence on abundance index estimates. As such, sampling must coincide with the maximum abundance hours, avoiding dawn/dusk periods when the population are moving from, or to, night-time grounds.
Keywords:
Parapenaeus longirostris,
Bottom trawling,
Diel variability,
behavioural rhythms,
GAM,
Portuguese continental waters
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Oral Presentation
Topic:
4. FISHERIES, AQUACULTURE AND BIOTECHNOLOGY
Citation:
Rodríguez-Climent
S,
Sonderblohm
CP,
Fonseca
P,
Erzini
K and
Campos
A
(2016). Diel variation in the deep-water rose shrimp Parapenaeus longirostris (Lucas, 1846) catches off the Portuguese south coast.
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00073
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Received:
15 May 2016;
Published Online:
02 Sep 2016.
*
Correspondence:
Dr. Sílvia Rodríguez-Climent, IPMA-Instituto Português do Mar e da Atmosfera, Lisboa, Portugal, silviarodriguezcliment@gmail.com