The southern king crab Lithodes santolla is a commercially very valuable decapod of the family Lithodidae. Although it is widely distributed on the Argentinian continental shelf, individuals are mainly grouped into a few sectors. The most productive argentine king crab fishery in the country has been developed since 2004 on the Patagonian Central Sector (PCS), which comprises the San Jorge Gulf and the continental shelf region between 44º and 48 ° S. In the present study the evolution of catches and fishing effort are investigated, abundance indices are estimated, and the spatial distribution, the population structure, and the main events of the king crab life cycle are characterized, based on data collected by scientific observers onboard the crab commercial fleet and those obtained during research surveys.
From the beginning of the fishery, the management has been based on the so-called “3S principle” (size, sex, and season). Only males above 110 mm can be landed during the fishing season, and the definition of such a season was modified as the knowledge about the life cycle increased over time. The exclusive use of traps to catch this species and the full coverage of the fleet by observer program agents were also established, essential to properly monitor and assess this commercial activity. In fact, between October 2004 and June 2016, direct records were obtained on 85% of the fishing hauls, making this marine fishery one with the highest coverage in the country.
Until the 2009-2010 season, the only licensed crabber vessel landed an average of 700 tons, whereas during the period 2010-2011 to 2015-2016 since four vessels were authorized and catches increased until reaching a peak of 3,700 tons. Null catches were very scarce, demonstrating the efficiency of the traps to catch these crustaceans. As a result of changes that occurred over time in the number of authorized vessels, new jurisdictions opened to fishing, and resource abundance, variations in the spatial distribution of fishing effort were verified. It was mainly concentrated on the Southern Sector of the shelf (46º-48º S), but the North Sector (44º-46º S) and Chubut within the San Jorge Gulf, were important fishing grounds during the last period. Abundance indices on both Total Catch and Legal-sized Males were estimated from fishing data using a General Linear Model (GLM). Declining trends in the indices along the last four fishing seasons were determined, and a drop of 28% for the legal males was found. Considering the spatial dispersion of the fishery since the 2011-2012 season, these indices seemed to properly reflect the actual variations in population abundance.
In order to understand the population dynamics of this species, different aspects of its live cycle were studied. The catch retained in the traps was characterized by a predominance of males, although in recent seasons a decreasing trend in the proportion of specimens of this sex, mainly those of commercial legal size, and a reduction in the average carapace length were observed. These changes are possibly associated with the selective nature of this fishing. The sex ratio varies during a fishing season, because the catchability decreases during the carapace molt, therefore the proportion of mature females is lower in November-December, while those of males decreases in autumn. The mandatory use of three escape rings in the trap since the 2014-2015 season markedly reduced the capture of females and non-legal males. An estimate of the carapace length at which 50% of individuals are retained by these traps (LC50%) was performed. In addition, the proportions of mature, non-ovigerous females, as an indicator of the lack of available male mates, were analyzed and the length of the first maturity was estimated. The temporal variability associated with the beginning of both the molting and mating processes was considered to be negligible. The carapace wear and the color of the ventral surface of the carapace were used to determine the aging of the carapace and to describe the inter-molt period.
The crab spatial distribution covered nearly all the Patagonian Central Sector, and it was characterized by stable aggregations over time. During systematic research surveys using traps, the highest densities of males were identified on the San Jorge Gulf, mainly within Santa Cruz jurisdiction, and also two other important aggregations were located on the shelf to the north and south 46ºS parallel. These results were consistent with the estimated abundance indices, and the areas where fishing effort is concentrated, as well as where king crab catches occurred during the trawl surveys to evaluate shrimp and hake carried out. A possibly linking, transitional zone showing very low abundances were identified among the above-mentioned shelf aggregations. The size distribution of both sexes was similar and remained stable. The average sizes increased towards the central region of the San Jorge Gulf and towards the east on the shelf. The evaluation surveys carried out with fishing vessels allowed to obtain highly accurate data over a short period of time.
The first data on southern king crab in the PCS came from research surveys targeting hake and prawn by using trawling nets. The hake surveys covered the period 1997-2014 and a wide sector. On the other hand, five shrimp cruises including an area of interest for southern king crab studies which was restricted to the San Jorge Gulf were selected. The presence of southern king crab increased over the years due to a gradual expansion on the shelf. The 8ºC and 10ºC isolines generally delimited the sectors where maximal catches were taken, whereas they were scarce or even null between Camarones and Rawson, where bottom temperatures exceed 11ºC in summer. The estimated densities in both of the mentioned surveys were consistent, having a maximum in 2011, without correlation in the following years, for this reason, this event would be associated with changes in catchability.
The increase in southern king catches determined the appearance of the first signs of overexploitation, such as the reduction in the abundance indices over the main fishing sectors, and decreases in both the proportion of males as well as their average length. In order to monitor the fishery, four population indicators are proposed to be annually analyzed: percentage of males, the average carapace length of males, the percentage of non-ovigerous mature females (≥ 70 mm), and the percentage of females having a full clutch. This work provides a baseline analysis on key biological-fishing issues aspects of southern king crab in PCS, which were studied since the very beginning of targeted fishing of this species, with the aim of achieving sustainable exploitation and adaptive ecosystem management.