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Distribución y estructura poblacional del cangrejoHalicarcinus planatus (Brachyura, Hymenosomatidae) en el Canal Beagle, Tierra del Fuego America and has a sub-Antarctic distribution, including the peri-Antarctic islands and New Zealand. In this paper, we report the abundance and population structure of H. planatus for a year in the intertidal zone of Bahía Brown, Beagle Channel, Tierra del Fuego. Moreover, we show variations in the population structure in other beaches and environments of the channel. In contrast with males, female density showed monthly variations and was higher during austral summer months. High densities of H. planatus could be related with low mortality during the larval stage and oceanographic features of Bahía Brown, probably enhanced by the presence of both culture and mussel beds, substrates where settlement would occur. The populational sex ratio was biased towards females only in summer, while the operational sex ratio was biased towards males during all periods studied, suggesting strong competition among males for mating. In turn, males were present only in the intertidal zone of Bahía Brown and Macrocystis pyrifera but were absent in the other studied intertidal areas.This is perhaps related to the lack of shelter in the subtidal zone of Bahía Brown. Recruitment occurs in two different periods, summer (JanuaryFebruary) and autumn-winter (May-June), which corresponds with the two larval cohorts of H. planatus in the Beagle Channel.Key words: Abundance, sex ratio, sexual maturity, recruitment, sub-AntarcticResumen.-El cangrejo Halicarcinus planatus es el único himenosomátido que habita el extremo austral de Sudamérica, con una distribución subantártica, incluyendo las islas periantárticas y Nueva Zelanda. En este trabajo, se presenta información sobre la abundancia y estructura poblacional de H. planatus en el intermareal de Bahía Brown, Canal Beagle, Tierra del Fuego durante un año. Además, se muestran las variaciones en la estructura poblacional en otras playas y ambientes del canal. A diferencia de los machos, la densidad de hembras mostró fluctuaciones mensuales en el intermareal de Bahía Brown y fue mayor durante los meses de verano. Las altas densidades de H. planatus podrían relacionarse con la baja mortalidad de estadios larvales y las características oceanográficas de la bahía, probablemente aumentado por la presencia de bancos y cuerdas de cultivo de mejillones, sustratos donde ocurriría el asentamiento de H. planatus. La proporción de sexos poblacional estuvo sesgada hacia las hembras solo en verano, mientras que la proporción sexual operativa se sesgó hacia los machos durante casi todo el período estudiado, sugiriendo una fuerte competencia entre machos por el apareamiento. A su vez, los machos solo estuvieron presentes en el intermareal de Bahía Brown y en los grampones de Macrocystis pyrifera pero no se encontraron en el resto de los intermareales estudiados, probablemente relacionado con la ausencia de refugio en el submareal de Bahía Brown. El reclutamiento existiría ...

Captura incidental de invertebrados en bancos de pesca de vieira patagónica: un caso de estudio con datos obtenidos por el Programa Observadores a Bordo

The present article is the first formal record of pelagic swarms and beach strandings of the squat lobster Munida gregaria in the Beagle Channel, southern South America. To describe size composition and natural diet of beach strandings and swarms, samples from strandings were taken surveys were carried out to describe both, horizontal distribution and school metrics of swarms. Pelagic swarms and beach strandings were composed of 100% individuals of the morph gregaria of M. gregaria. Mean size of males and females in both, beach strandings and swarms, were similar. The diet of pelagic M. gregaria was composed mainly of crustaceans, unicellular algae and small macroalgae, sediment and particulate organic matter. Swarms of pelagic Munida took the shape of extended layers of varying density and height. Their positions in the water column were also variable: swarms were found at different depths, from the subsurface layer to near the sea bottom. Occasionally, some large swarms occupied most of the water column. In the Beagle Channel, SW winds during spring tides could be a forcing factor for M. gregaria strandings. The absence of shoaling animals in the Beagle Channel between 1997 and 2002 and the recent occurrence of swarms are coincident with their appearance in other locations in Patagonia. In consequence, we hypothesize that the factor influencing the formation of swarms must be acting at a regional scale.

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The crab Halicarcinus planatus is the only hymenosomatid crab that inhabits the southern tip of South America and is the only decapod species that reproduces twice a year in the Beagle Channel. In this article, we study the moult cycle in the field (moult frequency, analysis of size frequency distribution) and linked it with growth studied in the laboratory (absolute and per cent growth increment, Hiatt function). Hiatt functions were similar for males and females. Moult frequency was seasonal: in early austral spring and in austral summer. In females, the pubertal moult is the terminal moult, whereas males continue moulting after attaining the size of morphometric maturity. Moult increment was highly variable. The relationship between absolute moult increment and crab size was described by a quadratic function. Per cent growth increment decreased with size, and relationships were different for each sex: linear for females and quadratic for males. Seven and eight modal groups explained the size frequency distributions for females and males from the field, respectively, and revealed the existence of two cohorts of recruits per year. Further modal analysis was mainly hampered by the high variability of size increment that could make any moulting individual fall in its own or one of two following modal groups. The antagonism between growth and reproduction was evident in small males. We hypothesize that the terminal pubertal moult is an advantageous feature that allows females to maximize their investment in reproduction after their terminal moult, which allows this species to have two spawnings per year.