REISS ET AL.: POPULATION DYNAMICS OF LOLIGO OPALESCENS IN THE SOUTHERN CALIFORNIA BIGHT CalCOFI Reports, Vol. 45, 2004 INVESTIGATING ENVIRONMENTAL EFFECTS ON POPULATION DYNAMICS OF LOLIGO OPALESCENS IN THE SOUTHERN CALIFORNIA BIGHT CHRISTIAN S. REISS, MICHAEL R. MAXWELL, AND JOHN R. HUNTER ANNETTE HENRY Southwest Fisheries Science Center California Department of Fish and Game National Marine Fisheries Service, NOAA 8604 La Jolla Shores Drive 8604 La Jolla Shores Drive La Jolla, California 92037 La Jolla, California 92037 [email protected] ABSTRACT Using fishery-dependent and fishery-independent data on the distribution of paralarvae, juveniles, and adults, we assess the effects of the transition from the 1997–98 El Niño to the 1999 La Niña on the abundance, distri- bution, and production of market squid Loligo opalescens in southern California. Percentage occurrence of par- alarvae within the CalCOFI survey of the Southern California Bight declined during El Niño, indicating that little production occurred during El Niño winters, and abundance from trawl surveys was low during the summer following the 1997–98 El Niño along the coast from Point Conception to Vancouver on the shelf and slope. Mean growth rates of L. opalescens declined with increasing temperature in the month of hatch, and hatch month temperature explained 67% of the variability in mean growth during the transition from El Niño to La Niña conditions (1998 and 1999). Using the observed temperature-dependent growth rates, we developed the first age-based, temperature-dependent population model for L. opalescens. Population dynamics were dramatically influenced by cumulative mortality, and the results pre- dict that a bimodal recruitment period should be found Figure 1. Plot of annual squid catch between 1965 and 2001 for Monterey with periods of 5 and 7 months and that the second peak (circles), Southern California Bight (triangles), and the total of both (squares). Arrows indicate fishery decline during El Niño periods followed by should be narrower than the first peak. rapid recovery. INTRODUCTION noteworthy given that L. opalescens is short lived (~ 6 The market squid Loligo opalescens supports one of the months; Jackson 1994; Butler et al. 1999; although Spratt most valuable commercial fisheries in central and south- (1979) hypothesized that L. opalescens lived more than ern California with a value of more than $41 million a 18 months) and evidently semelparous (Knipe and year (CDFG 2001). The total annual catch has increased Beeman 1978; Macewicz et al. 2004). The rapid changes exponentially since the 1970s (fig. 1; CDFG 2001). Most in apparent abundance in the fishery have led to con- of the increased catch (80%) has occurred in the Southern siderable speculation regarding the strategy of squid to California Bight (SCB), and there is uncertainty whether ride out the presumably harsh El Niño conditions. Given the increase is related to increasing abundance (Vojkovich the rapid recovery of squid, some investigators have ar- 1998). Rapid declines in squid catch occur during El gued that the population is displaced to the north or off- Niño years, and it has been suggested that the environ- shore during El Niño (CDFG 2001). If so, abundance ment may be partially responsible by influencing the of squid might increase with latitude during El Niño, availability, population size, and recruitment success of or abundance or presence might increase offshore. this animal (McInnis and Broenkow 1978). Alternatively, during El Niño conditions squid may move More remarkable than the near collapse of the fish- to deeper waters where the environmental conditions ery during El Niño periods is the rapid recovery of the necessary for proper development of eggs may be found, catch to pre-El Niño levels within a year or two fol- making the squid unavailable to the fishery and to those lowing (fig. 1; Maxwell et al., 2004). This is especially marine mammals that forage near shore (Lowry and 87 REISS ET AL.: POPULATION DYNAMICS OF LOLIGO OPALESCENS IN THE SOUTHERN CALIFORNIA BIGHT CalCOFI Reports, Vol. 45, 2004 Carretta 1999; Roberts and Sauier 1994). Both hy- Center triennial summer trawl survey conducted be- potheses suggest that squid become unavailable to the tween Point Conception, California, and Vancouver fishery, thus allowing the population to rebound rapidly Island, Canada, to examine changes in distribution; (2) after an El Niño. They also suggest that harsh El Niño para-larval abundance data from winter and spring conditions are overcome by redistributing the popula- CalCOFI surveys conducted in the Southern California tion to more favorable refuge habitats, potentially main- Bight to examine changes in productivity; and (3) growth taining production. rate data determined from mature adult squid collected The observed patterns of fishing success during and from the SCB commercial fishery to examine the envi- following the El Niño may also reflect demographic ronmental control on recruitment to the fishery. After processes occurring in the planktonic or juvenile stages, evaluating these hypotheses, we present an age-based where the environment may change vital rates. Of the temperature-dependent (TDGR) model of squid growth many environmental variables that may affect survival and develop a simple population dynamics model using and recruitment of squid, temperature is thought to be TDGR to drive the population growth rates. We com- one of the dominant mechanisms, because it can reflect pare predictions derived from this model to the overall different oceanographic regimes (Lynn et al. 1982; patterns of occurrence of squid in southern California Waluda et al. 2001) and because temperature is related as a starting point for directed studies on L. opalescens. to physiological processes (Forsythe 1993). For exam- ple, growth rates are strongly influenced by temperature METHODS in many squid species including Loligo spp. (Hatfield et al 2001; Forsythe 1993). Using laboratory data, Forsythe Fishery Data (1993) showed that under conditions of ad libitum food, Loligo opalescens were sampled at biweekly intervals growth rates of squid increased with increasing temper- from the commercial catch landed at ports within the atures. Jackson et al. (1997) used field data to show that SCB between 15 November 1998 and 01 July 2000. On growth rates increased in warmer conditions for a lolig- each date, mantle length (ML, in millimeters), gonad inid common to neritic waters of the northwest Gulf of weight (GW, in grams), maturity stage (immature or ma- Mexico. In contrast, Jackson and Domeier (2003) showed ture), and sex (male or female) were obtained from a that age and size of L. opalescens declined with increas- subsample of the catch. A further subsample of 15–60 ing water temperatures and increased with upwelling (an animals each month provided statoliths for age and index of secondary production) in the SCB. Based on growth analysis. All samples from the SCB were pooled these data, Jackson and Domeier (2003) suggested that and given equal weight in all analyses. the size difference of squid observed in the diets of sea Several studies have validated the daily deposition of lions could be explained by variability in the tempera- rings in laboratory reared L. opalescens (Jackson 1994a; ture-dependent growth differences in the Southern Butler et al. 1999). In the laboratory, statoliths were California Bight. They did not examine the effects on processed following Butler et al. (1999). Briefly, statoliths population size and recovery. were mounted on microscope slides, ground on 400 µm Growth rate variability can potentially affect the syn- paper until the primordium was visible. Under magni- chrony of cohorts. Mixing of different cohorts on the fication (x100–400) daily rings were counted using trans- spawning grounds may occur as a result of the strong mitted light to determine the age of each squid. Hatch temperature dependencies that cause fast growing co- date was determined by subtracting the age at capture horts to catch up to slow growing cohorts (Grist and de from the date of capture. Clers 1998, 1999), and bimodality in recruitment pulses Preliminary analysis showed that most of the animals may result. Thus, the question remains whether the collected as part of this study were mature adults (98%; apparent rapid repopulation of L. opalescens is a function Maxwell et al. 2004), so we defined growth rate (GR) of environmental variability operating directly on the of mature animals in the fishery as biological and physiological processes of the population GR = mantle length (mm) / age (d) (1) or whether it is simply reflecting changes in availability. In this study, we examine the hypotheses regarding Thus, growth rate as estimated here was the average value the changes in abundance associated with El Niño con- over the entire life of the animal. ditions in an effort to determine whether squid “ride Many growth functions and growth models exist for out” El Niño conditions by changing spatial distribu- squid (Jackson et al. 1997; Yang et al. 1986). We feel tion, or whether changes in demographic rates could be that the simple relationship we propose is adequate for responsible for the observed changes and abundance in this exploration for two reasons: (1) We are not pre- the fishery. To do this, we use (1) juvenile abundance dicting the size at any age during the paralarval or juve- and distribution data from the Northwest Fishery Science nile period when a more appropriate growth function 88 REISS ET AL.: POPULATION DYNAMICS OF LOLIGO OPALESCENS IN THE SOUTHERN CALIFORNIA BIGHT CalCOFI Reports, Vol. 45, 2004 would be necessary; and (2) under any contemporary model, faster (slower) growth rates would result in younger (older) animals recruiting to the fishery. We then correlated the mean growth rate of monthly co- horts, assigned using retrospective hatch-date distribu- tions to examine the relationship between growth rate of survivors caught in the fishery and environmental variables, as indexed by hatch month sea surface tem- perature (SST) at Scripps Pier, La Jolla, California.