Influence of upwelling conditions on feeding habits and trophic position of planktophagous fish

The effect of upwelling conditions on the trophic level of four planktofagous fish (Gadiculus argenteus, Capros aper and juveniles of Micromesistius poutassou and Merluccius merluccius) on the continental shelf off Northwestern Spain was assessed by coupling stomach content analyses and isotopic data. The study was performed during two consecutive years characterized by contrasting oceanographic conditions during the summer season; the first year (2012) had an intense and extended upwelling period, while the second (2013) had an intermittent and weak upwelling season. Sampling was carried out in autumn, after the summer upwelling, along a transect parallel to the coast on the continental shelf (130-180m depth), following the main upwelling gradient towards the Northeast. In addition to stomach content sampling and isotopic analyses of fish muscle, the isotopic signature of benthic detritus and sub- superficial phytoplankton were also indentified to characterize the basal isotopic level. We hypothesize that differences in isotopic ratios under these contrasting upwelling intensities possibly originate from both changes in the isotopic ratios of basal levels and shifts in the feeding habits of planktofagous species. Basal ratios of carbon and nitrogen in benthic detritus showed neither interannual variability nor relation with the upwelling gradient. However, particulate organic matter (POM) had significantly higher δ15N and δ13C during the first year. Isotopic signature of the four species was significantly different among years but followed different patterns, only related to POM basal levels in the case of C. aper. The ratio of 15N indicated that the four fish species considered had a trophic level of two, standing approximately 6 points over the basal levels and probably predating on consumer species. Stomach sampling evidenced that euphausiids constituted the majority of diet in G. gadiculus and juvenile M. merluccius and M.poutassou in both years. Euphausiid species would act then rather as consumers than predators, as a preliminary analysis of their isotopic signature confirmed (for the species Nyctiphanes couchii and Meganyctiphanes norvergica). While basal differences in isotopic ratios exist, our analyses suggest that shifts in feeding habits may have a major role in determining species isotopic positio

Agrupaciones demersales y bentónicas de los fondos arrastrables del mar de alborán (mediterráneo occidental)

The composition and abundance of megabenthic fauna caught by the commercial trawl fleet in the Alboran Sea were studied. A total of 28 hauls were carried out at depths ranging from 50 to 640 m. As a result of a hierarchical classification analysis four assemblages were detected: (1) the outer shelf group (50-150 m), characterised by Octopus vulgaris and Cepola macrophthalma; (2) the upper slope group (151-350 m), characterised by Micromesistius poutassou, with Plesionika heterocarpus and Parapenaeus longirostris as secondary species; (3) the middle slope group (351-640 m), characterised by M. poutassou, Nephrops norvegicus and Caelorhincus caelorhincus, and (4) the small seamount Seco de los Olivos (310-360 m), characterised by M. poutassou, Helicolenus dactylopterus and Gadiculus argenteus, together with Chlorophthalmus agassizi, Stichopus regalis and Palinurus mauritanicus. The results also revealed significantly higher abundances in the Seco de los Olivos seamount, probably related to a higher food availability caused by strong localised currents and upwellings that enhanced primary production. Although depth proved to be the main structuring factor, others such as sediment type and food availability also appeared to be important. Differences between shelf and slope assemblages could be in part related to a greater dependence on benthic resources in the former and a higher use of planktonic resources in the latter.Se estudia la composición y abundancia de la fauna capturada por barcos comerciales de arrastre en el mar de Alborán. Se llevaron a cabo 28 lances a profundidades comprendidas entre los 50 y los 640 m. El análisis de clasificación jerárquica dio como resultado cuatro agrupaciones: (1) plataforma externa (50-150 m) caracterizada por las especies Octopus vulgaris y Cepola macrophthalma, (2) talud superior (151-350 m), caracterizada por Micromesistius poutassou, con Plesionika heterocarpus y Parapenaeus longirostris como especies secundarias, (3) talud medio (351-640 m) caracterizada por M. poutassou, Nephrops norvegicus y Caelorhincus caelorhincus, y (4) el monte submarino Seco de los Olivos (310- 360 m), donde M. poutassou, Helicolenus dactylopterus y Gadiculus argenteus son las especies más características, junto a otras como Chlorophthalmus agassizii, Stichopus regalis y Palinurus mauritanicus. Los resultados revelan una abundancia de megafauna significativamente mayor en Seco de los Olivos, probablemente relacionada con una mayor disponibilidad de alimento provocada por las fuertes corrientes y afloramientos que tienen lugar en la zona, y que generan un aumento en la producción primaria. Aunque la profundidad constituye el principal factor estructurante de las comunidades, también son importantes el tipo de sedimento y la disponibilidad de alimento. Las diferencias encontradas entre las comunidades de plataforma y de talud podrían estar en parte relacionadas con una mayor dependencia de los recursos bentónicos en la primera, y un mayor aprovechamiento de los recursos planctónicos en la segunda.

Spatio-temporal variability in the cannibalistic behaviour of European hake Merluccius merluccius: the influence of recruitment abundance and prey availability

Cannibalistic behaviour of European hake Merluccius merluccius was studied through the analysis of 49 836 gut contents belonging to individuals from 6 to 82 cm in total length (LT). Samples were collected every autumn between 1993 and 2009. The results showed that the consumption of conspecific individuals was consistent over space and time. The abundance, spatial distribution patterns and LT structure of M. merluccius recruits were the main variables involved in M. merluccius cannibalism. A geographical pattern was found since increasing cannibalism was observed in areas of recruit aggregations. The LT spectrum of recruits in autumn was also a key factor and dependent on the spawning period. When adults spawned from late spring to summer, an increasing cannibalism trend was found in autumn, due to the ideal size structure of the prey (M. merluccius recruits) for predators. Depth was also a significant variable, and a cannibal peak was detected at depths ranging between 50 and 200 m, coinciding with a spatial overlap of predator (pre-adults) and prey (recruits). The cannibalistic behaviour of M. merluccius began at c. 12 cm, although 75% were mid-sized individuals between 16 and 30 cm. Additionally, 90% of prey-hake were recruits <18⋅5 cm. Some biological and ecological aspects such as growth rate, spawning period, predation and competition interactions and the effect of some oceanographic events on M. merluccius cannibalism are also discussed.En prens

Empleo de modelos de series temporales para la mejora de la calidad de las series históricas en pesquerías

Abstract: The so-called baca trawl fleet of the port of A Coruña working in the VIIIc ICES Division Area has been used since 1985 as the tuning fleet for the analytical assessment of the southern stock of hake Merluccius merluccius (L., 1758). The historical series of cpue dates from 1983 to 2000 with data missing from 1998. From January 1999, landings were estimated by means of monthly samplings in port. The predictions of the arima model fit the 1983-1997 series, and estimates of the transfer function model from the 1983-2000 series made it possible to complete the series, and to detect errors and possible changes in the orientation of the fleet. The comparison of the two models confirms the series’s stability over time, validating the estimates made in the last two years.Empleo de modelos de series temporales para la mejora de la calidad de las series históricas en pesquerías La flota de baca del puerto de A Coruña que opera en el área VIIIc del CIEM se usa desde 1985 como flota de calibración en la evaluación del stock sur de merluza Merluccius merluccius (L., 1758). Se dispone de una serie histórica mensual de cpue (1983-2000), excepto para 1998, año del que no se posee información. Desde enero de 1999 los desembarcos han sido estimados a partir de muestreos mensuales en lonja. Las predicciones efectuadas por el modelo arima ajustado a la serie 1983-1997 y las estimaciones del modelo de función de transferencia, considerando la serie 1983-2000, han permitido completar la serie, así como detectar errores o cambios en el comportamiento de la flota. La comparación de ambos modelos confirma la estabilidad de la serie, validando las estimaciones efectuadas en los dos últimos años.Instituto Español de Oceanografí

Using time series methods for completing fisheries historical series

The so-called baca trawl fleet of the port of A Coruña working in the VIIIc ICES Division Area has been used since 1985 as the tuning fleet for the analytical assessment of the southern stock of hake Merluccius merluccius(L., 1758). The historical series of cpue dates from 1983 to 2000 with data missing from 1998. From January 1999, landings were estimated by means of monthly samplings in port. The predictions of the arima model fit the 1983-1997 series, and estimates of the transfer function model from the 1983-2000 series made it possible to complete the series, and to detect errors and possible changes in the orientation of the fleet. The comparison of the two models confirms the series's stability over time, validating the estimates made in the last two years.La flota de baca del puerto de A Coruña que opera en el área VIIIc del CIEM se usa desde 1985 como flota de calibración en la evaluación del stock sur de merluza Merluccius merluccius (L., 1758). Se dispone de una serie histórica mensual de cpue (1983-2000), excepto para 1998, año del que no se posee información. Desde enero de 1999 los desembarcos han sido estimados a partir de muestreos mensuales en lonja. Las predicciones efectuadas por el modelo arima ajustado a la serie 1983-1997 y las estimaciones del modelo de función de transferencia, considerando la serie 1983-2000, han permitido completar la serie, así como detectar errores o cambios en el comportamiento de la flota. La comparación de ambos modelos confirma la estabilidad de la serie, validando las estimaciones efectuadas en los dos últimos años.Instituto Español de Oceanografí

To what extent is fishing effort affecting the trophic level of the demersal communities at small scale? Preliminary results of a new food web indicator

Due to the complexity of benthic and demersal ecosystems, to be really helpful, a food web indicator should summarize, in a single number, a variety of complex processes that are otherwise hard to apprehend. The present work assesses the direct impact of bottom fishing gears on the trophic level (TL) of benthic and demersal communities dwelling in the continental shelf of the southern Bay of Biscay. Using a spatio-temporal approach at small scale, we tackle two main objectives: 1) to evaluate temporal trends in mean TL at community scale per sampling rectangle (5 x 5 miles grid resolution), and 2) to assess to what extent these trends are related to fishing effort. Two types of data are involved in the analyses: 1) TL data (mean trophic level by sampling rectangle), and 2) VMS data (number of fishing days by sampling rectangle). TL data were calculated using stomach contents sampled for demersal fish species during the Demersales surveys (IBTS), carried out every autumn in the southern Bay of Biscay since 1983, and combined with data from Fishbase and local references for those species which lacked empirical data. We only considered species that appeared in a minimum of 5 years and were consistently well identified along the whole historical series, in order to keep consistency in the community spatiotemporal sampling. Preliminary results point to a significant increase in TL in most of the sampling rectangles, suggesting that the trend observed globally is spatially homogeneous. A clear relationship between fishing effort and TL was also identified, particularly in those areas where fishing effort (mainly otter trawl fisheries) has been decreasing in recent years

Trophic mechanisms underlying bentho-demersal community recovery in the north-east Atlantic

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