BULLETIN OF MARINE SCIENCE. 89(3):699–716. 2013 http://dx.doi.org/10.5343/bms.2012.1063 VARIABILITY IN THE GROWTH PATTERNS OF LOXECHINUS ALBUS ALONG A BATHYMETRIC GRADIENT ASSOCIATED WITH A FISHING GROUND Carlos Molinet, Cecilia A Balboa, Carlos A Moreno, Manuel Diaz, Paulina Gebauer, Edwin J Niklitschek, and Nancy Barahona ABSTRACT Here we assess the growth pattern variability of the urchin Lochechinus albus (Molina, 1782) in a fishing ground where it was distributed between 0 and 100 m depth. The bathymetric gradient was divided into four strata, and urchin samples were collected for growth estimations. Images were used to characterize the urchin population along this bathymetric gradient, and historical records for the urchin fishery at this fishing ground were examined. Four algorithms were used to model growth, and the Akaike’s Information Criteria was used to determine the best model fit. Among the four bathymetric strata, both size and age composition, and the growth patterns of L. albus differed significantly.I n the shallowest stratum, urchins were smaller and younger than in the deeper strata. Urchins from 5 to 15 m depth displayed greater initial growth rates compared with urchins from 25 to 100 m depth; however, growth decelerated faster at 5–15 m depth than at deeper habitats. Based on results, we hypothesize that the growth pattern of L. albus observed in the shallowest stratum represents a case of age (size) truncation due to fishing, which requires further study. Since the end of the 1990s, the sea urchin Loxechinus albus (Molina, 1782) has supported the greatest edible sea urchin fishery in the world (Vásquez et al. 1984, Guisado and Castilla 1987, Moreno and Vega 1988, Andrew and O’Neill 2000, Vásquez 2001, Moreno et al. 2006, 2011, Kino and Agatsuma 2007, Pérez et al. 2010). This fishery began in Chile in the 1940s, but steady growth did not occur until the mid-1970s (Moreno et al. 2006). The fishing pressure led to a crisis in 2001, which led to the establishment of a management plan that took effect in 2005 M( oreno et al. 2006). Loxechinus albus is one of the key herbivores of the coastal ecosystems off Chile (Vásquez et al. 1984, Guisado and Castilla 1987, Moreno and Vega 1988, Vásquez 2001, Kino and Agatsuma 2007, Pérez et al. 2010). Its geographic distribution spans the southern cone of South America, from Isla Lobos in Peru (06°55.5´S, 80°42.5´W) in the Pacific to the central zone of Argentina (37°35´S) and the Malvinas/Falkland Islands (51°40´S) in the Atlantic (Schuhbauer et al. 2010). In the fjords and channels of southern Chile, around 90% of the L. albus population inhabits between 0 and 20 m depth (Inostroza et al. 1983, Moreno et al. 2011), even though its bathymetric distribution has been described as from the intertidal down to 340 m depth (Larraín 1975). This extended distribution range reported for L. albus is linked to the spe- cies’ capacity to feed on drifting macroalgae (Castilla and Moreno 1982, Contreras and Castilla 1987), which reaches deep habitats via physical transport mechanisms (Vetter and Dayton 1998, 1999). Bulletin of Marine Science 699 © 2013 Rosenstiel School of Marine & Atmospheric Science of the University of Miami 700 BULLETIN OF MARINE SCIENCE. VOL 89, NO 3. 2013 It is reasonable to expect differences in individual growth rates for urchins in deeper strata associated with energetic limitations related to lower food availability (Ebert et al. 1999, Wing et al. 2003, Schuhbauer et al. 2010). In addition, but to a lesser extent, changes in temperature, salinity, and dissolved oxygen over a bathy- metric gradient also may have metabolic effects on sea urchins (Siikavuopio et al. 2007, 2008, Schuhbauer et al. 2010). Along with the trophic, metabolic, and reproductive factors that determine bathy- metric differences in L. albus individual growth patterns, it is reasonable to expect fishing effects as suggested byS chuhbauer et al. (2010). These effects can lead to density-dependent and trophic consequences observed in benthic species (e.g., Orensanz 1986), and compensatory growth as observed in fishes (e.g., Ali et al. 2003). Evolutionary responses induced by the selective pressure of the fishery have been ob- served in other species, such as Atlantic cod, Gadus morhua, Linnaeus, 1758 (Swain et al. 2007), and white seabream, Diplodus sargus (Valenciennes, 1830) (Pérez-Ruzafa et al. 2006). The fit of growth models in areas where larger individuals have been selectively re- moved by fishing results in truncated size and age structures, and can cause errone- ous estimates and interpretations of growth curves, with management consequences for the fishery (Götz et al. 2008, Hsieh et al. 2010). In addition to the range of factors that may affect the growth patterns of urchins, comparison of these patterns based on available literature is complicated by the diversity of models used to analyze observed growth in different populations and depth strata of interest (Grosjean 2001). Among the three published studies of L. albus growth, Gebauer and Moreno (1995) applied the von Bertalanffy model, Schuhbauer et al. (2010) selected the von Bertalanffy model, and Flores et al. (2010) selected the Tanaka model among three other models. Studies by Gebauer and Moreno (1995; 39°26´01˝S, 73°12´57˝W, inter- tidal) and Schuhbauer et al. (2010; 52°03´06˝S, 59°44´15˝W, 0–15 m depth) were car- ried out on urchins from unfished areas, while Flores et al. (2010; between 44°00´S and 44°30´S, <19 m depth) studied urchins from populations affected by intense fish- ing since the 1980s (e.g., Moreno et al. 2006, 2011). Here, we look at the variability in growth patterns of the only known population of L. albus distributed between 0 and 100 m depth (Moreno et al. 2011, Molinet et al. 2012), an area which is also actively fished. The discovery of this bed allowed us to study and compare growth patterns along a bathymetric gradient, where the population is fished down to 60 m depth, regardless of the physiological and legal restrictions that limit shellfish divers to 20 m depth. Thus, we compared the size and age structure of L. albus among four bathymetric strata, and fit four alternative growth models to data from each stratum, using an information criterion to select the best model. Finally, we compared the apparent growth patterns within the differ- ent strata, formulating and assessing, in an exploratory manner, various hypotheses that explain the differences observed. Materials and Methods Study Area.—The study area is characterized by a circular bathymetric depression down to 110 m depth located to the south of Chiloé Island (43°10´S, 73°39´W), near Quellón in northwestern Patagonia (Chile; Fig. 1A), the port with the highest landings of the L. albus MOLINET ET AL.: GROWTH OF LOXECHINUS ALBUS ALONG A BATHYMETRIC GRADIENT 701 Figure 1. Study area in the southern Chile. (A) The study area is circled; the arrow indicates the location of Quellón, the principal port for sea urchin landings in the study area. (B) The enlarged study area, with black squares indicating the position of transects carried out with the Seabotix LBV 200 remote operated vehicle. Gray contour lines indicate the depth (m) isobaths and negative numbers indicate the depth of the isobaths. The area surrounded by broken lines represents the estimated area occupied by the patch of Loxechinus albus, as reported by Molinet et al. (2009) and C Moreno (Universidad Austral de Chile, unpubl data). fishery (Molinet et al. 2011). Here, a bed of L. albus was found with heterogeneous densities down to 100 m depth (Molinet et al. 2009, 2012, Moreno et al. 2011), occupying an area of about 6 km2 (Fig. 1B). Additionally, landing records from the Instituto de Fomento Pesquero show a fishing ground that extends from the western limit of the bed toward the center of the hollow. Sample Collection.—Samples were collected from four strata along a bathymetric gra- dient: 5–15, 25–45, 50–70, and 75–100 m. The shallowest stratum was selected based on the greater frequency of catch up to 15 m deep in the records of the fishery. The other three 702 BULLETIN OF MARINE SCIENCE. VOL 89, NO 3. 2013 strata were arbitrarily selected. In the three deepest strata, samples were collected by towing a “chain sweep” trawl, modified from Campagna et al. (2005), a distance of 100 to 200 m. The trawl was 1 cm mesh size, 1-m2 mouth, and 2 m in length. In the shallowest stratum, samples were collected by divers due to the uneven nature of the sea floor that rendered trawling im- possible. Divers searched for and collected all individuals from the sampling area to complete the number of urchins per size class required for reading growth rings. From each depth stratum, 20–30 urchins per size class were collected, with intervals of 5 mm per size class where possible. In the case of the deepest samples, given that this is the only reported bed of L. albus below 50 m depth, only 1000 individuals were taken per stratum so as to not significantly affect abundance, which has been estimated around 54,000 individuals (Molinet et al. 2009). To obtain a representation of the bed size distribution, images were taken with a SeaBotix Little Benthic Vehicle LBV200 over 29 transects, each 100–150 m long and 0.3–0.8 m wide, to measure and count a sample of urchins within the study bed. The LBV200 was equipped with a Micron Nav USLB tracking system, with a transductor fixed to a mast and a GPS an- tenna configured to record the geographic position every 1 s.