Pyura Chilensis (Ascidiacea) in Southern Chile: the Urgent Need to Establish Marine Reserves
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Revista Chilena de Historia Natural 68: 107- 116, 1995 Over-exploitation of Pyura chilensis (Ascidiacea) in southern Chile: the urgent need to establish marine reserves Sobre-explotaci6n de Pyura chilensis (Ascidiacea) en el sur de Chile: la urgente necesidad de establecer más reservas marinas ANDREW R. DAVIS Department of Biological Sciences, University of Wollongong Wollongong NSW 2522, AUSTRALIA ABSTRACT I use data on the intertidal abundance and size frequency of one of Chile's commercially exploited invertebrate species, the '"Piure" (Pyura chilensis) to argue that intertidal populations ofthis species are dramatically over-exploited. Here, I compare a population at the Mehuin Marine Reserve with four exploited sites. Densities of P. chilensis were more than 3 orders of magnitude higher in the Marine Reserve compared to these sites. Within the reserve, I recorded an average density of 576±83 mˉ² (±sem, median=400), an average size (wet weight) 15.3±1.5 g (±sem) and a maximum size of 112 g. Outside of the reserve, individuals rare! y achieved a size of more than 20 g. As P. chilensis became reproductive at between 12 and 13 g, this corresponds to only around 6% of individuals in the harvested populations achieving sexual maturity and compares with an average value of 37±4.8% (±sem) within the reserve. A snapshot examination of patterns ofrecruitment within the reserve, revealed that juveniles only recruited successfully in the presence of adult conspecifics, indicating that recovery of exploited populations is likely to be slow. This study represents one of a number in Chile that underscore the dramatic effects of human harvesting on rocky intertidal communites. I urge Chilean authorities to heed the warnings of their marine scientists, as there is an urgent need to fund the establishment of protected marine areas. Key words: ascidian, density, harvesting, size-frequency, recruitment RESUMEN Utilizando datos de abundancia y frecuencia de tallas del invertebrado Pyura chilensis ("Piure"), obtenidos durante este estudio comparativo de las poblaciones de Ia Reserva Marina de Mehuin y poblaciones de localidades vecinas explotadas, se puede argumentar que las poblaciones intermareales de esta especie están dramaticamente explotadas. Las densidades de P. chi!ensis en Ia Reserva fueron 3 6rdenes de magnitud mayores que en los sitios explotados. En Ia Reserva registre una densidad promedio de 576±83 mˉ² (±cern), un peso humedo promedio de 15,3±1 ,5 g (±cern, mediana=400), y un peso maximo de 112 g. En tanto que fuera de Ia Reserva los individuos rara vez presentaron un peso mayor a 20 g. La talla de madurez sexual ( 12-13 g) Ia alcanz6 s6lo el 6% de los individuos de las poblaciones explotadas, en cambia Ia proporci6n de individuos reproductivos dentro de Ia Reserva fue de 37% (±4,8% ). Un rapido examen de los patrones de reclutamiento en Ia Reserva, revelo que los juveniles reclutaron exitosamente s6lo en presencia de individuos conespecfficos, lo que estaria indicando una baja recuperaci6n de las poblaciones de esta especie en los sitios explotados. Este estudio es uno más de los varios realizados en comunidades intermareales rocosas chilenas que muestran el efecto dramatico de la intervenci6n humana. Debido a ello, hago un llamado a las autoridades chilenas a que presten atenci6n a la urgente necesidad de establecer nuevas áreas marinas protegidas. Palabras clave: ascidia, densidad, explotaci6n, frecuencia de tallas, reclutamiento INTRODUCTION is unique among temperate zone rocky shores in the high level of human Temperate marine ecosystems are renowned exploitation to which it is subject. This is for their diversity and productivity perhaps even more surprising when one (Suchanek 1994). In Chile, the striking considers the coasts generally exposed diversity and biomass of organisms on the nature, rough water and few harbours." rocky coast is matched by the degree to (cited in Castilla 1976). Among the species which these resources are exploited. After that Paine considered to have undergone a visit to Chile in 1974, R.T. Paine noted " ... local reductions in abundance, and that ... "I believe that the Chilean shoreline significant changes in distribution ... " he (Recibido cl 22 de junio de 1994; aceptado el 3 de diciembre de 1994) 108 DAVIS lists both species of intertidal ascidian; myself to the study of intertidal populations. Pvura chilensis Molina 1782 and P. The effects of human exploitation have not 1 praeputialis (Heller 1878) • been examined for this species (but see More than 11 genera of marine inver- footnote 2) and previous studies of P. tebrates and 7 genera of intertidal algal are chilensis have focussed largely on indivi- exploited on the coast of Chile and dual growth and reproductive biology of frequently a number of species within each populations on Chile's northern coast genus are harvested (Castilla 1976). The (Gutierrez & Lay 1965, Cea 1973). I am high level of exploitation of these organisms not aware of any current data on distribution and the ensuing changes in the structure and abundance of this species. Indeed and dynamics of intertidal communites Santelices et al. ( 1977) in a study which when humans are excluded, is truely they described as "the first quantitative dramatic (Moreno et al. 1984, 1986, Durán description of intertidal communities along eta/. 1987, Durán & Castilla 1989, Godoy the Pacific South American Coast" make 2 & Moreno 1989, ). no mention of Pyura chilensis. To what Here I focus attention on the commercially extent this may reflect over-exploitation of exploited ascidian Pyura chilensis. This this species is not clear. Other references species commonly occurs in the low to the distribution and abundance of P. intertidal and subtidal zone (depth to 70 m) chilensis appear to have, on occasions, from the Island of Chiloe (Golfo de Ancud - 42"S) to southern Perú (Van Name 1945, 1 It is noteworthy that this later species is referred to as Pyura Brattstrom & Johanssen 1983). Intertidal bradleyi by Van Name in his treatise on North and South and subtidal populations of this animal are American Ascidians (Van Name I945). 'Moreno CA (1994) Ecologfa de Choromytilus chorus en un commercially exploited (CA Moreno, per- ambiente intermareal: un estudio de largo plazo. p. 99. sonal communication). Here, I restrict Resumenes. XIV Jornadas de Ciencias del Mar, Puerto Montt. .. Fig. 1: Location of the four study sites in southern Chile. Mehuin is the only non-harvested site. Ubicaci6n geográfica de los cuatro sitios de estudio en el sur de Chile. Solamente Mehuin tiene una Reserva Marina. OVER-EXPLOITATION OF PYURA CHILENSIS 109 confused this species with Pyura between the siphons and wet weight (blotted praeputialis (Santelices 1989). This pyurid dry) for these individuals. I then regressed is also found in the rocky intertidal zone, wet weight with the linear measurements but has a distinctive upright morphology to determine the best estimator of ascidian and is restricted to rocky shores in the biomass. vicinity of Antofagasta, northern Chile (Paine & Suchanek, 1983). Estimates of density and size frequency Here I examine intertidal populations of P. chilensis at three harvested sites and Densities and size frequencies were within a Marine Reserve. Specifically, I estimated in two ways. At harvested sites, compare densities and size frequencies at 10 horizontal transects were laid at the harvested sites with those in the Reserve. I tidal height at which I expected to find P. also examine patterns of recruitment in P. chilensis. Each transect was 5 m in length chilensis within the Marine Reserve. and any P. chilensis occurring 10 em above or below the transect line were counted. Hence each transect corresponded to 1 MATERIALS AND METHODS square metre of rocky substratum. These transects were placed haphazardly in areas Stud'Y sites spanning a range of exposure to wave action; from exposed vertical rock faces to The density and size frequency of P. relatively sheltered pools. Usually, the chilensis was examined at four sites. The maximum width of 10 haphazardly selected Marine Reserve at Mehuin (39° 24' S; 73° individuals of P. chilensis within each 13' W) was closed to the general public and transect was also determined. Measure- was the only non-harvested site. Monthly ments were made to the nearest mm. This size frequency data was collected at this methodology could not be applied at the site in January through April, 1994. The Marine Reserve. At this site, quadrats three sites from which organisms were measuring 10 x 10 em were placed in pools harvested lie to the north and south of in the zone in which P. chilensis occurs. In Mehuin and were selected because of their these pools, the animals siphons were similarity in wave exposure and mix of covered by water and hence they continued species to the Marine Reserve (Fig. 1). to feed. This rendered the bright red lining Pichicuyin is the next headland directly of the siphons easy to discern and therefore north of the Marine Reserve and is separated the number of individuals were easily from the Reserve by approximately 1 km of distinguished and counted. Ideally sandy beach. Two adjoining headlands densities should be estimated using quadrats comprise the sites to the south; San Ignacio placed at random; hence, my use of pools at (39° 50' S; 73° 24' W) is separated from the this site may bias my estimates of density, more northerly headland at Playa Rosada by although casual observations in areas of approximately 2 km of sandy beach. All the Reserve that lacked pools indicated harvested sites were sampled in March, 1994. that this was not the case. Densities in a total of 51 quadrats were recorded in the Determining the best predictor of biomass Marine Reserve, corresponding to a total area of just over half a square meter. Pyura chilensis is often irregular in shape Size frequencies of P.