Recursos Marinos y Pesquerías

Evaluación de métodos de muestreo de Maja brachydactila [Working paper]

06/07/05 ,

Antonio Corgos y Juan Freire. Evaluación de métodos de estimación de abundancia y etructura poblacional de la centolla brachydactyla en sustratos sedimentarios. Working paper (v.1, Jul 2005) [pdf]

Un buen método de muestreo para la obtención de datos sobre
la abundancia y la estructura espacial y demográfica de una población de un
organismo marino sería aquel que permita 1) obtener estimaciones (absolutas o
relativas) insesgadas de abundancia, 2) representar adecuadamente la estructura
espacial y demográfica y 3) obtener un tamaño de muestra suficiente con un
coste limitado para el análisis de las distribuciones de frecuencias de talla y
otros parámetros demográficos. Se llevó a cabo una evaluación de métodos de estimación
de abundancia y estructura poblacional de la centolla, Maja brachydactyla, en la Ría de A Coruña (NW Spain). Se
realizaron muestreos con artes de enmalle (miños), arrastre (bou de vara), buceo,
y nasas. El enmalle presentó problemas importantes por los daños en los
ejemplares muestreados y en el propio aparejo y por la escasa resolución
espacial que proporciona sobre la distribución de los organismos a pequeña
escala. El muestreo mediante buceo no fue viable por los problemas logísticos
derivados de la climatología y oceanografía de la zona de muestreo, que reducen
la capacidad de prospección (frecuentemente la visibilidad es reducida, la
turbidez es elevada, y presenta un elevado hidrodinamismo). Se compararon las
capturas obtenidas en muestreos mensuales con nasas y beam trawl en una misma
área durante un ciclo anual (Junio 1998 – Junio 1999). Las capturas con nasas
(tanto absolutas como por unidad de esfuerzo, CPUE) fueron mucho más elevadas, aunque
el patrón estacional fue similar con ambos métodos. La relación entre las
capturas con nasas y beam trawl se ajustó al modelo CPUE rnasas (ejemplares·nasa-1)
= 7.007·CPUE arrastre (ejemplares·lance-1)exp(0.56) (R2 = 0.64), lo que indicaría
que la CPUE con
nasas desciende más lentamente que la obtenida con arrastre, que pondría de
manifiesto la existencia de hiperestabilidad. Con el arrastre se capturó una mayor
proporción de ejemplares de muy pequeña talla (los ejemplares menores de 60 mm de longitud de
caparazón, LC, representaron el 23% del total de capturas con arrastre y sólo
el 0.7% con nasas), y pocos ejemplares grandes (LC>130 mm), especialmente
adultos (la capturabilidad relativa de ejemplares adultos fue del 12% de la
correspondiente a la de juveniles). Se estimó el área de pesca de una nasa
(para juveniles de LC?70 mm) en 1440
m²; que equivaldría a un radio de acción de 21.4 m. Para obtener las
mismas capturas, el coste (en días de muestreo) del muestreo con arrastre sería
casi 4 veces superior al realizado con nasas.

Moult cycle of Maja brachydactila [Working paper]

06/07/05 ,

Antonio Corgos y Juan Freire.

Patrones relacionados con el sexo y el tamaño corporal en el ciclo de muda de una población de aguas someras de la centolla Maja brachydactyla en la Ría de A Coruña. Working paper (v.1, Jul 2005) [pdf]

Versión en castellano (v1 Jun 2005) [pdf]

The moult cycle of a shallow-water population
of Maja brachydactyla was analysed in
the Ria de A Coruña (NW Spain) during two complete yearly cycles. The juveniles
presented a seasonal pattern with moult peaks in spring (March-May) and in
autumn (September-October), although crabs with moulting activity throughout
the winter were also observed. During the spring peak and during the summer the
percentage of males moutling was higher than that of the females. The males presented a terminal moult cycle
which was longer (from April to September, with a peak in July) and occured
earlier than that of the females (from June to August, with a peak in August).
The small-sized juveniles (carapace length, CL?85 mm) were found with moulting
activity throughout the whole yearly cycle, whereas the larger-sized juveniles
(CL >85 mm)
presented a clear seasonal pattern. The smaller males (CL?130 mm) carried out the terminal moult in June, whereas
the larger males (CL?140 mm) delayed it until September. In the case of the
females no differences were detected in the timing of the terminal moult
according to size.

Versión en castellano en la continuación

Continue reading Moult cycle of Maja brachydactila [Working paper]…

Spatial dynamics of a local population of Maja [Conference]

22/06/05 ,

A Corgos, N Sánchez & J Freire (2005). Dynamics of the small-scale spatial structure of a local population of the spider crab, Maja brachydactyla .Sixth International Crustacean Congress, University of Glasgow, Scotland, 18-22 July 2005.
POSTER (pdf)
WORKING PAPER v.1, Jun 2005 (pdf)

Mark-recapture analysis of growth in Maja brachydactyla [Working paper]

13/05/05 ,

A. Corgos, M. P. Sampedro, E.
González-Gurriarán and J. Freire. Growth at moult, intermoult period
and moulting seasonality of the spider crab Maja
brachydactyla: combining information from mark-recapture and experimental
studies. (Working paper, v.1, May 2005) [pdf]

An analysis of growth at moult (for
both the prepubertal and terminal moults), moulting seasonality and the
intermoult period in the spider crab Maja squinado in the Ría de A Coruña (NW
Spain) was carried out based on a mark-recapture experiment. Crabs between 70
and 130 mm carapace length (CL) undergo a mean increase at moult of 32.4% from
their pre-moult size. Generalised Linear Models (GLMs) were used to construct
growth models, employing a combination of information from the mark-recapture
study and other previous studies performed in both laboratory and extensive
culture, to estimate the effects of the biological variables and the study
method. No differences were found in the growth rate between males and females.
However, the effects of the study method, the premoult CL and the interaction between
them were significant. The smallest-sized crabs undergo a greater increase in
size in the laboratory and culture studies, while the largest individuals
undergo greater growth in the field. The mean intermoult period estimated for
prepubertal moults in the field ranged from 50 to 86 days, which is similar to
the 84.7 days observed in the laboratory study. The  prepubertal moults occurred primarily in
spring and autumn in the field, while under culture conditions, the crabs
moulted mainly in the spring. The intermoult period for terminal moults was
estimated to be around 90 days, slightly lower than the value of 104 days
obtained in the laboratory. The terminal moult took place generally in summer
(June-September) both at sea as well as in culture experiments. The intermoult
period of juveniles at sea was highly variable, and some of the specimens did
not moult for more than 5 months.

Reproductive strategies of Maja brachydactyla [Working paper]

13/05/05 ,

Antonio Corgos, Patricia Verísimo and Juan Freire.
Estacionalidad de la muda terminal y la migración reproductiva en la centolla, Maja brachydactyla: evidencias de
estrategias de apareamiento alternativas. Working paper (v.1, Jul 2005) [pdf]

Versión en castellano (v.1, May 2005) [pdf]

Timing and synchronisation at individual and population levels of the processes related to the terminal moult, gonad maturation, accumulation of energy reserves and migration in the spider crab Maja brachydactyla are analyzed. Also, the intra- and intersexual variability is established. Two hypotheses are tested to explain the temporal and population variability:1) physiological hypothesis: males and females begin migration when they reach the appropriate physiological stage (the optimum level of energy reserves); and 2) mating opportunities hypothesis: the timing of the migration maximise mating opportunities and mate quality. Our results show that males carry out the terminal moult before females, the former having a peak in July and the latter in August. The onset of gonad maturity in females occurs between two and three months after they have reached morphometric maturity (starting in October), coinciding with the period prior to and during the mating migration to deep waters. In an analysis of the spermathecae of primiparous females, it was found that practically no mating activity occurred in shallow waters, while the first copulations took place in the migration corridor. However probably most of the mating activity occurs in deep mating grounds. Males reached gonad maturity prior to morphometric maturity. On average, males started migrating on 8 October, while females began migration on 16 October, which would corroborate previous evidence pointing to an earlier starting date for males. No differences were observed in the physiological status (muscle, gonad and hepatopancreas relative mass) between migrating and non-migrating crabs, which is a clear indication that the physiological hypothesis does not hold true for this species. A higher percentage of postmoult crabs were caught in the migration corridor than in shallow waters. The physiological condition improved over time in both males and females in the specimens caught in the shallow area as well as in the migration corridor. Therefore, the crabs that start migrating first did so in poorer physiological condition. Moreover, the early migrators had a significantly lower mean size than the late migrators. Our results would suggest that variability within populations and between sexes is related to the different reproductive strategies and not to a physiological limitation. Thus, the poor-quality males (with a reduced competitive ability) would migrate at the beginning of the season (prior to or in synchronisation with the first females), whereas the high-quality males might delay migration until they have accumulated a high level of reserves, because they would be able to displace low-quality males at mating habitats.

Versión en castellano en la continuación

Continue reading Reproductive strategies of Maja brachydactyla [Working paper]…

Distribution and habitat use in coastal decapods [Working paper]

10/05/05 ,

A Pallas, B García-Calvo, A Corgos, C Bernárdez, J
Freire. Inter-specific comparative analysis of distribution and
habitat use patterns of benthic decapod crustaceans in shallow waters. Working paper (v.1, May 2005). [pdf]

ABSTRACT: Coastal areas have been widely considered as
nurseries for many marine species. New approaches to this concept take into
account interactions among environmental variables and ecological variations
related to geographical location, as well as complex life cycles of marine
invertebrates. We present a comparative approach to assess the relevance of
environmental variables on the determination of patterns of distribution and
habitat use of benthic decapod species in coastal areas. We hypothesize that
this approach allows us to infer processes originating these patterns and to
identify the main habitat use models. An intensive fine-grain sampling design
was used to take into account the environmental gradients occurring at
different spatial scales (defined by substrate type, depth, exposure and
geographical location) in a temperate oceanic bay (Ria de A Coruña, Spain). A
high proportion of juveniles were found in most populations, but the results do
not allow us to generalize the idea of coastal areas as potential nurseries,
except for few species with a marked spatial segregation between juveniles and
adults. Larval transport seems to be the main process regulating mesoscale
distribution patterns, while microscale distribution responds to a complex interaction
among different processes, i. e. habitat selection at settlement, differential
mortality among habitats, post-settlement dispersal and ontogenetic habitat
shifts. Sandy substrates showed low-diversity communities dominated by hermit
crabs. In rocky bottoms, variability in spatial patterns was mostly related to
substrate type and geographical location. Caridean shrimps showed higher
densities on flat rock surfaces, with similar juvenile and adult patterns.
Anomuran species occurred mainly on cobbles. Distribution patterns of
brachyurans varied among species, but did not change greatly from juveniles to
adults.

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