Length-weight relationships of 15 mesopelagic shrimp species caught during exploratory surveys off the Canary Islands (central eastern Atlantic)

INTRODUCTION

Mesopelagic shrimps live between the surface and 1000 m depth, and the most abundant families are Acanthephyridae, Benthesicymidae, Ciferidae, Oplophoridae, Pandalidae, Pasiphaeidae, Penaeidae and Sergestidae (Landeira and Fransen 2012Landeira J.M., Fransen C.H.J.M. 2012. New data on the mesopelagic shrimp community of the Canary Islands region. Crustaceana 85: 385-414. https://doi.org/10.1163/156854012X626428, Vereshchaka et al. 2019Vereshchaka A.L., Lunina A.A., Sutton T. 2019. Assessing deep-pelagic shrimp biomass to 3000 m in the Atlantic Ocean and ramifications of upscaled global biomass. Sci. Rep. 9: 1-11. https://doi.org/10.1038/s41598-019-42472-8). Many mesopelagic decapod crustaceans show diel vertical migrations (DVMs) associated with predation avoidance, ascending towards surface waters at night to feed and descending to depths during the day to hide from predators (Irigoien et al. 2004Irigoien X., Conway D.V., Harris R.P. 2004. Flexible diel vertical migration behaviour of zooplankton in the Irish Sea. Mar. Ecol. Prog. Ser. 267: 85-97. https://doi.org/10.3354/meps267085, Torres et al. 2018Torres A.P., Reglero P., Hidalgo M., et al. 2018. Contrasting patterns in the vertical distribution of decapod crustaceans throughout ontogeny. Hydrobiologia 808: 137-152. https://doi.org/10.1007/s10750-017-3414-x).

Over the years, the estimation of mesopelagic shrimp biomass from oceanographic campaigns has been influenced by a catchability problem of the fishing gears, since the sample size is usually not very large and the DVMs can significantly affect the estimated biomass in the water column (Vereshchaka et al. 2019Vereshchaka A.L., Lunina A.A., Sutton T. 2019. Assessing deep-pelagic shrimp biomass to 3000 m in the Atlantic Ocean and ramifications of upscaled global biomass. Sci. Rep. 9: 1-11. https://doi.org/10.1038/s41598-019-42472-8). Biomass is normally estimated from acoustic data and primary production studies (Irigoien et al. 2014Irigoien X., Klevjer T.A., Røstad A., Martinez U., Boyra G., Acuña J.L., Bode A., Echevarria F., Gonzalez-Gordillo J.I., Hernández-Leon S., Agusti S., Aksnes D.L., Duarte C.M. Kaartvedt S. 2014. Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nat. Comm. 5: 1-10. https://doi.org/10.1038/ncomms4271) because the information available on the ecology and biology of these species, and particularly on growth, is very scarce.

Frequently, the lack of biological data on crustacean species with low or without fishing interest at regional level is partially compensated by using standardized data available on global databases such as SeaLifeBase.org (Palomares and Pauly 2012Palomares M.L., Pauly D. 2012. FishLifeBase. Available at: www.sealifebase.org), where it is possible to obtain the maximum length and weight reported for some species, the depth range and the geographical distribution. However, such data are only useful for a general approach, not for establishing any particular characteristic of the species at a more local or regional level. Unfortunately, in the particular case of mesopelagic shrimps, the biological information available is almost nil in the specialized literature, particularly for the central eastern Atlantic, with the exception of a few taxonomic monographies (Zariquiey-Alvarez 1968Zariquiey-Álvarez R. 1968. Crustáceos decápodos ibéricos. Inv. Pes., 32: 1-510., De Grave and Fransen 2011De Grave S., Fransen C.H.J.M. 2011. Carideorum catalogus: The recent species of the Dendrobarachiate, Stenopodidean, Procarididean and Caridean shrimps (Crustacea: Decapoda). Zoo. Med., Leiden, 85: 195-588.), first records, faunal lists and latitudinal/vertical distributions of species (Quiles et al. 2001Quiles J.A., González J.A., Santana J.I. 2001. New and little known Dendrobranchiata and Caridea of the Canary Islands (Crustacea, Decapoda). Bol. Inst. Esp. Ocean. 17: 7-13., Muñoz et al. 2012Muñoz I., García-Isarch E., Sobrino I., et al. 2012. Distribution, abundance and assemblages of decapod crustaceans in waters off Guinea-Bissau (north-west Africa). J. Mar. Biol. Ass. UK, 92: 475-494. https://doi.org/10.1017/S0025315411001895, Vereshchaka et al. 2019Vereshchaka A.L., Lunina A.A., Sutton T. 2019. Assessing deep-pelagic shrimp biomass to 3000 m in the Atlantic Ocean and ramifications of upscaled global biomass. Sci. Rep. 9: 1-11. https://doi.org/10.1038/s41598-019-42472-8

Species life-cycle parameters are required for the proper management of fishing resources, but also for estimating biomass fluxes between trophic levels and assessing the role of each group within the marine ecosystem (Couce-Montero et al. 2021Couce-Montero L., Christensen V., Castro J.J. 2021. Simulating trophic impacts of recreational fishing scenarios on two oceanic islands using Ecopath with Ecosim. Mar. Env. Res. 169: 105341. https://doi.org/10.1016/j.marenvres.2021.105341). Knowledge on the length-weight relationships (LWR) can be used to gather information of species growth patterns, estimate condition index and analyse growth variations on temporal or spatial scales between populations/stocks (González-Acosta et al. 2004González-Acosta A.F., De La Cruz Agüero G., De La Cruz Agüero J. 2004. Length-weight relationships of fish species caught in a mangrove swamp in the Gulf of California (Mexico). J. App. Ichth. 20: 154-155. https://doi.org/10.1046/j.1439-0426.2003.00518.x, Gerritsen and McGrath 2007Gerritsen H.D., McGrath D. 2007. Significant differences in the length-weight relationships of neighbouring stocks can result in biased biomass estimates: Examples of haddock (Merlangius merlangus, L.) and whiting (Merlangius merlangus, L.). Fish. Res., 85: 106-111. https://doi.org/10.1016/j.fishres.2007.01.004, Froese and Pauly 2015). In this study, 15 mesopelagic shrimp species caught around the Canary Islands (central eastern Atlantic) were analysed to estimate the LWRs and analyse their relative growth patterns.

MATERIAL AND METHODS

RESULTS

A total of 1210 specimens belonging to 15 species from 3 superfamilies (Oplophoroidea, Penaeoidea and Sergestoidea) were sampled and identified (Table 2). Five families were collected, Sergestidae being the most abundant in both number of species (eight) and individuals, following by Oplophoridae with three species.

The growth patterns of 15 species were described from the LWRs parameters (Tables 1 and 2). The mean values of the correlation coefficients were high, with a mean value of 0.944±0.0333. Systellaspis pellucida showed the worst correlation index, with a value of 0.874, while Allosergestes nudus showed the highest correlation index (R=0.994). In relation to growth, it was observed that 60% of the species analysed showed isometric growth (b=3; t-test, p>0.05), while 6.7% and 33.3% showed positive (b>3; t-test, p<0.05) or negative allometric growth (b<3; t.test, p<0.05). Allosergestes sargassi and S. pellucida showed the lowest values of the allometry coefficient range (b), while Systellaspis debilis showed the highest value.

DISCUSSION

This study provides the first estimation of the LWRs of 15 mesopelagic shrimp species caught in several exploratory surveys carried out in the Canary Islands area (central eastern Atlantic). This is the first study that gives growth information on these mesopelagic decapod crustaceans. Biological data on these crustaceans are scarce, probably because the fishing gears used in the exploratory campaigns of this mesopelagic community make the individuals suffer a significant deterioration of their structures, because their exoskeleton and appendages are very fragile. The deterioration of these structures of high taxonomic value reduces the number of samples, so in this study only individuals that allowed a reliable taxonomic identification were included.

It should be noted that although the LWRs of the 15 species are shown, only 53.3% had the necessary correlation values for reliable LWRs (greater than 0.95, the preferred significant level).

The sample conservation system is a subject under study because conservation dehydrates the tissues. In the case of crustaceans, Fazhan et al. (2021)Fazhan H., Waiho K., Jalilah M., et al. 2021. Effect of different measuring techniques, preservation methods and storage duration on the morphometric measurements of crustacean larvae. Mar. Biol. Res. 17: 98-105. https://doi.org/10.1080/17451000.2021.1900576, describe reductions of around 5% for larvae of Scylla olivacea and Macrobrachium rosenbergii, although they state that this low percentage of contraction is a result of their rigid chitinous exoskeleton. In our study, we did not evaluate the effect of conservation on the dehydration of these individuals, and because previous studies are not known, it is recommended that the estimated parameters for the 15 species be used as preliminary. In conclusion, the information provided for these 15 species contributes to knowledge of these species and allows for more accurate biomass estimations of them, which is vital for the sustainable management and conservation of mesopelagic shrimp populations. The information will also be a reference for future comparisons between populations from other areas of the region, for identifying stocks in the same area, or for determining changes in the growth pattern according to variations in climate parameters (Gerritsen and McGrath 2007Gerritsen H.D., McGrath D. 2007. Significant differences in the length-weight relationships of neighbouring stocks can result in biased biomass estimates: Examples of haddock (Merlangius merlangus, L.) and whiting (Merlangius merlangus, L.). Fish. Res., 85: 106-111. https://doi.org/10.1016/j.fishres.2007.01.004).