MARINE ECOLOGY PROGRESS SERIES Published January 12 Mar. Ecol. Prog. Ser.

Nursery ground, age structure and abundance of juvenile monodon on the continental shelf off central Chile

Ruben ~oa\Victor A. ~allardo~~~,Billy ~rnst~, Mario Baltazar3,Juan I. Canete3, Sandro Enriquez-Brionnes3

'Institute de Fomento Pesquero, Sede Zonal V-IX Regiones, Casilla 347, Talcahuano, Chile 'Centre EULA, Universidad de Concepcion, Casilla 156-C, Concepcion, Chile %epartamento de Oceanografia, Universidad de Concepcion, Casilla 2407-10, Concepcion, Chile

ABSTRACT' Newly settled individuals and juveniles of several benthic decapod spend their first months or years in nursery habitats different from those of the adult population. It was unknown if this was the case for the squat lobster Pleuroncodes monodon, an exploited which inhabits the continental shelf off central Chile, and of which 2 adult populations exist, the larger Achira (35' 10' S to 36' 15' S) and the smaller Biobio (36' 35' S to 36' 50' S). We report here the pres- ence of a large nursery area of the species, connecting both adult populations; the habitat is dominated by extensive sulphide microbial communities which fluorish at very low oxygen concentrations. We inferred the existence of northerly and southerly migration routes from the nursery ground to the adult populations, with juvenile year classes migrating as they advance through age classes. The juvenile population was composed of 2 year classes: 0 yr old (newly settled mdividuals) and 1 yr old juveniles; thus juveniles seemed to spend their first year of life in the nursery area. Total abundance was esti- mated as 3290 x lo6individuals (asymmetric 95 % confidence interval: 2153 to 7039 x lo6),although the above figure is a lower bound because the sampling did not cover the whole nursery area. We hypoth- esize that the Biobio adult population is not self-sustaining, but depends on a surplus production of juveniles from the Achira population.

KEY WORDS: Squat lobster Nursery grounds Population biology. Benthic environment. Continen- tal shelf Central Chile

INTRODUCTION Consequently, a necessary step towards understand- ing and monitoring recruitment in benthic crustaceans Recruitment strength is a major factor in determining is a study on the existence and location of nursery population dynamics of many fish and invertebrate grounds. stocks in marine environments. The interplay between The squat lobster Pleuroncodes monodon is a small high fecundity and variable mortality of the pelagic galatheid decapod found on the continental shelf off larval phase accounts for a large part of population central Chile and harvested by a trawling fleet. There fluctuations in abundance and structure (Fogarty et al. are 2 large adult populations, the northern Achira and 1991, Koslow 1992, Doharty & Fowler 1994). In some the southern Biobio (Bahamonde et al. 1986, Roa & benthic crustacean populations, including penaeid Bahamonde 1993) (Fig. I), which together support an shrimps and lobsters, juveniles spend their first months annual catch of about 4000 t. Recently, we reported the or years of life in nursery areas different from those of existence of juvenile aggregations south of the Achira the adult population (Garcia & Le Reste 1981, Phillips population, although the spatial scale of our sampling 1986, Bishop & Kahn 1991, Wahle & Steneck 1991, Pol- was too limited to identify the region as a nursery lock 1993, Rogers et al. 1993, Watson & Turnbull 1993). ground (Gallardo et al. 1994). However, those results

1Inter-Research 1995 Resale of full article not permitted Mar. Ecol. Prog. Ser. 116-47-54, 1995 -

The age structure of the juvenile fraction of the squat lobster population is rather unclear. Roa (1993) used 5 research surveys done between 1982 and 1991 to analyse the age structure of the adult Achira popula- tion. However, the author lacked information on juve- nile year classes, due to the fishing gear used and loca- tions sampled in those surveys. The purpose of the present study was to analyse data from a research sur- vey especially designed to cover the likely grounds of the juvenile fraction of the squat lobster population. We describe part of a large nursery area, provide esti- mates of abundance, disclose the age structure of early benthic and adult life, and describe the association of the squat lobster with oceanographic features and the presence of Thioploca spp. mats.

MATERIALS AND METHODS

Field and laboratory methods. Previous results from 1991 and January and March 1992 (Gallardo et al. 1994) showed that the main period of benthic settle- ment was April. Therefore, the survey for juveniles was performed in April 1993. The sampling was made onboard the 17 m RV 'Kay-Kay', using an Agassiz trawl (1m width at the mouth) towed at 2 knots and whose mesh size (4 mm at the codend) was adopted to retain even the smallest individuals. The likely juvenile grounds were inferred from the distribution of larger individuals, referred to as adults Fig. 1. Sampling region for juvenile squat lobsters Pleuron- for convenience. Adults inhabit 2 populations sepa- codes monodon. inside gridded area- quadrats randomly rated by ca 45 km, the northern Achira and the chosen for sampling; +: hauls m the exploration area; cobble- like areas hard untrawlable bottoms; shaded areas show the southern Biobio populations (Fig. I), of which the for- Achira (north)and Biobio populations mer is 4 to 5 times larger in terms of numeric abun- dance (Roa 1994). In 1991, the Achira population suggested that nursery grounds may exist south of the showed a clear spatial age structure, with smaller (i.e. adult populations, a hypothesis further supported by younger) individuals inhabiting the southern end of the fact that, in the Achira population, individuals the adult population, and larger (1.e. older) ones pop- show a clear latitudinal separation of year classes: ulating the northern end (Roa & Bahamonde 1993). younger individuals populate the southern areas (Roa Consequently, we inferred that the Achira population & Bahamonde 1993). was fed with juveniles from the south, and defined a The hypothetical nursery areas are located on a survey area between 36' 10' S and 36' 40' S (Fig. 1). region of intense seasonal upwelling (Arcos & Wilson The oceanic border of this area was determined on 1984),with very low dissolved oxygen concentrations, the basis of the presence of hard untrawlable bot- and whose bottom is covered by large microbial com- toms. The squat lobster has not been found on shal- munities (Thioploca spp.; Gallardo 1977). A trophic low coastal bottoms. Using this information we deter- relationship between the squat lobster and Thioploca mined a survey area of 1950 km2 (Area 1) for the spp. has been suggested (Gallardo 1977). Thioploca juvenile grounds of the Achira population (Fig. 1). A spp. bottoms may also serve as a refuge from preda- second smaller adult population, the Biobio popula- tion, given that juvenile squat lobster is one of the main tion, inhabits the region surrounding a submarine prey items of the hake Merluccius gayi (Arancibia canyon off the Biobio River mouth (Fig. 1). We as- & Melendez 1987),the most abundant demersal fish off sumed that this population would also be fed with ju- central Chile. However, a small-scale spatial associa- veniles from the south, and then determined a second tion between Thioploca spp. mats and juvenile squat survey area of 341 km2 (Area 2) inside the Gulf of lobster has not been demonstrated. Arauco (Fig. 1). Roa et al.: Nursery habitat of Pleuroncodes monodon 49

Both survey areas were gridded into 10 km2 quad- Effort corresponded to the area swept by the trawl rats, and a total of 44 quadrats were randomly chosen on each haul, which was calculated using initial and in Area 1 and 12 m Area 2. One haul was made in each final positions recorded by the GPS and Pythagoras' quadrat, assuming that abundance was homogeneous theorem corrected for latitudinal position (Sparre et al. within each one. When the sampling of both areas was 1989): finished, 7 additional hauls (locations chosen haphaz- ardly) were made to obtain information on the region surrounding the submarine canyon (Fig. 1). Initial and final positions of each haul were recorded with a (2) Global Positioning System (GPS,Trimble Trans Pack I, where LTl and LTi are initial and final latitude, LNi ±3 m measurement error). On each haul, the total and LN2 are initial and final longitude, and the factor catch of lobsters was fixed with 10 % formaldehyde in 1852 x 60 converts from degrees to meters. This quan- seawater and the presence or absence of Thioploca tity corresponded to distance travelled but also to area spp. filaments was recorded. Together with trawling, swept because the trawl mouth had a width of 1 m. bottom water samples were taken in some quadrats In the absence of specific information, the catch- (22 in Area 1, 5 in Area 2, and 3 m the exploration ability coefficient (q)was set equal to 1. This value area). Water temperature was recorded in situ. A sam- means that all individuals within the path swept by the ple of water was immediately fixed for Winkler titra- trawl (and only those individuals) were caught by the tion, and the rest of the water was stored for salinity trawl. This assumption can be supported by the fact measurement. that no herding effect,which causes the capture of For each haul the total catch in numbers was mea- individuals from outside the swept area (q > I), has sured. Individuals were separated according to sex been described for benthic crustaceans. In fishes, the (visual observation of ovigerous setae), and carapace herding effectoccurs when the bridles and otter doors length (CL, from the eye socket dorsally along a line of the approaching trawl herd fish into the path swept parallel to the midline to the posterior edge of the cara- by the trawl, overestimating local density (e.g. see pace) was measured to the nearest 0.1 mm. When Krieger 1993).Also, escape of individuals from inside hauls yielded very abundant catches a subsample of the swept path (q< 1) seems unlikely with the trawl about 250 individuals was randomly taken to deter- approaching at velocities of 2 knots (ca 1.0 m s)and mine sex and CL. Some individuals, classified as unde- bottom currents on the order of a few cm. s (Marcus termined, could not be sexed with certainty due to Sobarzo, Centre EULA, pers. comm.), although we small size. lack information on natatory capabilities of small squat Estimation of total abundance. Total abundance (P) lobsters. was determined usmg the swept area method, which is To estimate mean CPUE in Eq. (I),we first analysed based on the assumption that catch per unit of effort the distribution of CPUE values. We expected that (CPUE) is linearly proportional to population density CPUE values would distribute log-normally, as is (populationabundance/area, P/A),with the coefficient usual in bottom trawling surveys (Pennington 1986, of proportionality being the catchability coefficient (q) McConnaughey & Conquest 1993), and they did (Ricker 1975; see also Ralston & Tagami 1992 and Roa (Fig. 3, and see 'Results').Thus, mean CPUE was esti- & Bahamonde 1993 for specific applications). The mated as the maximum-likelihood Finney-Sichel esti- expression for total abundance is mator (McConnaughey & Conquest 1993):

where y is the arithmetic mean of the variable (CPUE) expressed as natural logarithm [In(CPUE)],and G(r)is where a is the area swept by the trawl on any haul. In a function of the variance of ln(CPUE)of the form: this work, we express catch (C),and therefore abun- dance, as number of mdividuals. To delimit A, the coastal border was drawn at mid distance between hauls with absence and presence of squat lobsters; the where n is the number of hauls (all with positive oceanic border was drawn following the natural catches) and r = s2/2, where s2 is the variance of perimeter of hard untrawlable bottoms, and the north- ln(CPUE).G(r) is an infinite series, but we computed ern and southern borders were drawn as the smallest the Finney-Sichel estimator by using the first through distance between extreme hauls (Fig. 2). The area the seventh terms of the series, the seventh being enclosed by the perimeter was measured with a digital 1/100000 of the first term. The variance of the Finney- planimeter (LASICO 1280 Digitizer 42-C). Sichel estimator is given by (Pennington 1986): 50 Mar. Ecol. Prog. Ser. 116: 47-54, 1995

r^^Gulf of Arauco /s^7Gulf of Arauco

Fig 2 Delimitation of the nursery area (central shaded area) and its habitat characteristics (a)Oxylines (mgI"'), (b) Gulf of Arauco isotherms ('C); (c)isoha- !! lines; (d) presence (1) I fi-/ and absence (0)of Tho- ploca spp.

ric by virtue of the asymmetry of the log-normal distri- bution. Those limits are computed as (McConnaughey & Conquest 1993): where hauls with positive catches only are considered. A confidence interval for the abundance estimator (Eq. 1) was computed by assuming that the population area and the mean area swept were measured without error, so that A/q is a constant, and by noting that the standard error of the product of a constant and an esti- mate with error is the product of the constant and the where U and L stand for upper and lower limits, and standard error of the estimate [Seber 1982).Hence, the the Hstatistics (different for each limit) depend on the confidence interval of the abundance estimator would number of degrees of freedom and the variance of be given by the product of A/q and the confidence lim- ln(CPUE),and are tabulated in Land (1975). Our par- its of the Finney-Sichel estimator, which are asyrnmet- ticular combination of degrees of freedom and vari- Roa et al.: Nursery habitat of Pleuroncodes monodon 5 1

ance was not found in Land's (1975)tables, so a cubic interpolation was computed, as sug- gested by Land (1975).A measure of percent relative error in abundance estimation (RE) was given by REL = 100(P- L)/P and REU = 100(U - P)/P for lower and upper limits re- spectively. Analysis of population structure. An ex- ploratory plot showed that male and female size distributions, and those of the undeter- mined (usually very small) individuals, were very similar (Fig. 4a). Consequently, size dis- tribution data were pooled for further analy- sis. The global histogram was disclosed into year classes using the statistical-numerical CPUE (N m'2) algorithm MIX (Macdonald & Pitcher 1979) with criteria according to Roa (1993).

RESULTS

Abundance and delimitation of a nursery ground

There were no catches of juvenile squat lob- sters in the 12 hauls of Area 2; however, the catches in Area 1 and in the exploration area were highly successful (Table 1; 29 successful hauls out of 44). Unsuccessful hauls at the -2 -1 6 -1.2 -0 8 -0 4 0 0.4 0.8 1.2 1.6 coastal border in Area 1, together with the nat- Ln (CPU E) (N m") ural perimeter of the hard untrawlable bot- toms on the oceanic border and the northern Fig. 3. Pleuroncodes monodon. Distribution of catch per unit of effort and southern hauls, were used to delimit the (CPUE) values. (a) Untransformed values; (b)natural logarithm of CPUE nursery ground (Fig. 2). The northern and southern extreme hauls yielded high abun- dances, and thus those borders drawn did not enclose parameters of the distribution mixture (data pooled), the whole nursery area. CPUE values showed a typical we obtained mean sizes of 6.5 and 12.5 mm CL, with a log-normal distribution, with many values concentrated highly dominant representation, in terms of numeric on the smaller classes and a tail towards greater classes abundance, of the smallest year class (Fig. 4b, Table 2). (Fig. 3a).Logarithmic tranformation of the CPUE values yielded a more symmetrical (normal) distribution, al- though low sample size (29 successful hauls) precluded Table 1. Pleuroncodes monodon. Size of area, lobster density a completely clear picture (Fig. 3b).The relative error in (SD),and abundance on the nursery ground. Only successful abundance estimation was rather high for both limits hauls are considered (Table I),a consequence of low sample effort. Area 1 Exploration area

Population structure Degrees of freedom 28 6 Area (km2) 1376 The size structure of juvenile squat lobster in the Density (ind. m2) 2.39 (1.19) nursery ground showed the presence of 2 year classes Abundance (x lo6 ind.) 3290 for both sexes, with clear normal distributions for both Lower 2 5 % limit 2153 Relative error 34.5 (Fig. 4a). The sexually undetermined individuals Upper 97.5 % limit 7039 belonged to the normal distribution with the smallest Relative error 114.0 mean size (smallest year class). When estimating the 52 Mar. Ecol. Prog. Ser. 116: 47-54, 1995

2000 Oceanographic environment and 1800 - a Thioploca spp. 1600 - 1400 - Charts of oceanographic features of the 2 1200 - bottom water and the presence/absence of a Thioploca spp. in Area 1 are shown on Fig. 2. 0" 1000 -

variations, has a southerly flow (Brockmann et al. 1980, local variations in population variables. On the other Huyer et al. 1991). Consequently, larvae from the Bio- hand, oxygen profiles (Fig. 2a) indicate that, at the bio spawning stock would be advected south, espe- time of our survey, the oxygen-poor Peru-Chile cially early stages (zoeas). However, we did not find Countercurrent was reaching shallow depths. How- newly settled and juvenile individuals south of the Bio- ever, off central Chile, the border between this pole- bio population. Oceanographic and biological condi- ward subsurface water mass and the surface equator- tions were similar to those in Area 1, south of the ward oxygen-rich Subantarctic water is seasonally Achira population. Therefore, it might be that a signif- dynamic, being shallowest during spring-summer up- icant fraction of the larvae from the Biobio spawning welling and deepest during autumn-winter upwelling stock is lost from the population area, and that this relaxation (Fonseca 1986). It is this seasonal variation population depends on juveniles exported from the in oceanographic conditions, in particular, that may Achira spawning stock. affect bathymetric distribution of the whole juvenile The situation where juvenile crustaceans spend population, as suggested for adult squat lobster by their first months or years of life in nursery areas and Bahamonde et al. (1986). then migrate into adult populations has been reported The age structure of the juvenile populations in the for penaeid stocks (Garcia & Le Reste 1981) and lob- nursery ground allows completion of the age structure sters (Phillips 1990, Wahle & Steneck 1991). Nursery study done by Roa (1993).In Roa's work juvenile year areas in penaeids are normally estuaries (Garcia & Le classes were absent, but mean sizes of adult year Reste 1981), although seagrass habitats (Watson & classes and the parameters associated with the fitted Turnbull 1993) and shallow mudflats (Bishop & Kahn growth function predicted the existence of 2 juvenile 1991) have also been reported as nursery habitats. For year classes of mean sizes similar to those reported the Australian rock lobster Panulirus cygnus, nursery here (age 0: 4.4 mm CL predicted, 6.5 estimated here; grounds are inshore limestone reefs (Phillips 1990) age 1: 11.8 predicted, 12.5 estimated here; average of while for the American lobster Homarus americanus both sexes). Considering Roa's analysis and results of in Maine it is cobble substratum (Wahle & Steneck this study, it can be concluded that the age structure of 1991). The squat-lobster case provides another kind of the squat lobster population is made up of 8 age classes nursery habitat which is a reflection of the local fea- in males, and 9 in females. This can be regarded as the tures of the central Chile continental shelf: the exten- longevity of the species, under current exploitation sive microbial communities which flourish under con- conditions. ditions of very low oxygen concentrations. In every Our estimation of juvenile abundance in the nursery case, those different nursery habitats in penaeid ground can only be considered as preliminary, how- shrimps, rock lobster, American lobster, and squat ever. First, the sampling effort (26 successful hauls) lobster may share the property of providing food was low for a region of 1376 km2, resulting in high and/or refuge from the strong predation pressure ex- relative errors for the estimation. Second, we did not perienced by juveniles (Bishop & Kahn 1991, Wahle cover the whole nursery area, which leaves an 1992). Juvenile squat lobsters are one of the main unknown portion of the juvenile population to the prey items of the hake Merluccius gayi, the dominant north and the south of the assessment area unsampled. demersal fish off central Chile. Our results, then, lend Also, the existence of juveniles on the hard credence to Gallardo's (1977) hypothesis that there untrawlable bottoms to the oceanic border remains exists a strong association between the microbial unconfirmed. Our estimate of abundance can be con- communities and the stock of the squat lobster. Nev- sidered as a lower limit, in the sense that there are no ertheless, whether the environment of the microbial less than the amount estimated, with the specified communities is advantageous for juvenile lobsters degree of certainty. However, our discovery of the and/or does not differ in that respect with other habi- nursery ground and the border drawn for it makes it tats of the region remains unknown. possible to monitor recruitment strength on a yearly Oceanographical conditions of the bottom waters basis. An annual series of recruitment indices may correspond to those of the Peru-Chile Subsurface serve as a tool in evaluating the relationship, if it exists, Countercurrent, characterized by very low oxygen between recruitment strength and spawning stock of concentrations (Fig. 2a). Abundance and other popula- the squat lobster. The importance of such knowledge tion variables (size-age structure and sex proportion) has been forcefully stressed by Phillips (1989),despite were not correlated with oceanographic conditions on the costs involved in the construction of time series suf- a local basis (pairwise linear correlations, p > 0.05). ficiently long to attain statistical certainty. On the other Therefore it seems that the oceanographic conditions hand, without regard to the spawning stock-recruit- contribute to the environment in which the juvenile ment strength relationship, a series of recruitment squat lobster spends its benthic life, but do not cause indices remains a useful management tool for predict- 54 Mar. Ecol. Prog Ser. 116. 47-54, 1995

ing the level of future commercial catches, much in the Krieqer, K. J. (1993). Distribution and abundance of rockfish way that it is currently done for the rock lobster fishery determined from a submersible and by bottom trawling. in western Australia (Phillips 1986). Fish Bull. U.S. 91 87-96 Land, C. E. (1975).Tables of confidence limits for hnear func- tions of the normal mean and variance Sel Tables Math Acknowledgements We are grateful to Renato Qunones, Stat. 3, 385-419 Ignacio Paya and Ingo Werhtmann for reading the manuscript Macdonald, P. D. M., Pitcher, T. J. (1979) Age-groups from and makmg useful comments and criticisms Robert size-frequency data. a versatile and efficient method of McConnaughey kindly provided a copy of Land's (1975) analysing distribution mixtures. J. Fish. Res. Bd Can. 36 paper. Three anonymous referees made several comments 987-1001 that contributed to clarification of the text. This paper was McConnaughey, R. A, Conquest, L L. (1993) Trawl survey written on the basis of the final report of the study 'Reclu- estimation using a comparative approach based on log- tamiento Ecoloqico en el Langostino Colorado. Fase II', normal theory Fish. Bull. U.S. 91: 107-118 funded by the Subsecretariat of Fishing, Ministry of Economy Palma, S., Arana, S. (1990). Aspectos reproductivos del lan- of the Republic of Chile Soledad Zorzano, of the Subsecre- gostmo colorado (Pleuroncodes monodon) en la zona tariat, provided the impetus to do this work centro-sur de Chile. Estudios y Documentos Univ. Catolica 1/90 (Mimeo),Valparaiso Pennington, M. (1986). Some statistical techniques for esti- LITERATURE CITED mating abundance indices from trawl surveys. Fish. Bull. U.S. 84: 519-525 Ahumada, R., Rudolph, A, Martinez, V. (1983). Circulation Philips, B. F. (1986). Prediction of commercial catches of the and fertility of water in Concepcih Bay, Chile. Estuar. western rock lobster Panulirus cygnus. Can J. Fish. coast. Shelf Sci. 16. 95-105 Aquat. Sci. 43: 2126-2130 Arancibia, H., Melendez, R. (1987) AlimentaciOn de peces Philips, B. F. (1990). Estimating the density and mortality of concurrentes en la pesqueria de Pleuroncodes monodon juvenile western rock lobster (Panulirus cygnus) in nurs- Milne-Edwards. Investigacion pesq (Chile) 34 113-128 ery reefs. Can. J. Fish. Aquat Sci 47: 1330-1338 Arcos, D. F , Wilson, R. E (1984). Upwelling and the distribu- Pollock, D. E. (1993) Recruitment overfishing and resilience tion of chlorophyll a within the Bay of Conception, Chile. in spmy lobster populations. ICES J. mar Sci. 50: 9-14 Estuar. coast Shelf Sci. 18: 25-35 Ralston, S., Tagarm, D. T. (1992) An assessment of the Bahamonde, N., Henriquez, G., Zuleta, A,, Bustos, H , Baha- exploitable biomass of Heterocarpus laevigatus in the monde, R. (1986). Population dynamics and fisheries of mam Hawaiian islands. 1. Trapping surveys, depletion squat lobsters, Family Galatheidae, in Chile. Can. 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This article was submitted to the editor Manuscript first received: May 31, 1994 Revised version accepted: September 21, 1994