Influence of local-scale and landscape-scale habitat characteristics on California spiny lobster (Panulirus interruptus) abundance and survival

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1 CSIRO PUBLISHING Marine and Freshwater Research, 7, 58, Influence of local-scale and landscape-scale habitat characteristics on California spiny lobster (Panulirus interruptus) abundance and survival Thien T. Mai A,B,C and Kevin A. Hovel A A Department of Biology, San Diego State University, 55 Campanile Drive, San Diego, CA 92182, USA. B Present address: Section of Evolution and Ecology, University of California, Davis, One Shields Ave, Davis, CA 95616, USA. C Corresponding author. mai@ucdavis.edu Abstract. For many marine systems, little is known about the effects of habitat structure on ecological processes that dictate population dynamics. This study focused on the effects of habitat structure on behaviour, abundance, and survival of California spiny lobster (Panulirus interruptus Randall) in the Point Loma kelp forest, San Diego, California. Habitat characteristics were quantified in 4-m 2 landscapes to determine the role of shelter and understorey kelp characteristics at local (shelter) scales and landscape scales on lobster habitat use. A tethering experiment determined the effects of the presence of understorey kelp on lobster survival. At the shelter scale, lobsters preferred permanent shelters to ephemeral shelters, but did not respond to shelter size. At the landscape scale, lobster density increased with Pterygophora californica (stipitate kelp) density and decreased with Laminaria farlowii (prostrate kelp) density, but lobster density did not vary with shelter density or dispersion. Lobster size increased with P. californica density in two of three surveys, while lobster size did not vary with L. farlowii density. Lobster relative survival was higher in the presence of understorey kelp than when kelp was absent. We conclude that lobsters respond to habitat characteristics at local and landscape scales, and that understorey kelp has strong effects on lobster habitat use and survival. Additional keywords: behaviour, distribution, fragmentation, kelp forest, shelter, understorey kelp. Introduction Habitats are often heterogeneous (i.e. patchy) at a variety of spatial scales (Wiens 1976; Forman and Godron 1986; Pickett and Cadenasso 1995). In many terrestrial habitats such as forests and grasslands, patchiness at landscape scales strongly influences processes that determine the abundance, distribution, movement, and survival of organisms (Kareiva 1987, 199; Turner 1989; see reviews by Andren 1994 and Debinski and Holt ). In the marine environment, habitat patchiness is common in structured nearshore habitats such as seagrass meadows (Robbins and Bell 1994; Hovel and Lipcius 1), coral reefs (Sale and Douglas 1984; Syms and Jones ), and kelp forests (Dayton and Tegner 1984; Bologna and Steneck 1993). In seagrass habitats, where the majority of marine landscape-scale studies have been conducted, patch size and landscape configuration influence organismal growth (Irlandi 1994; Bologna and Heck 1999), survival (Irlandi et al. 1995; Hovel and Lipcius 1; Hovel 3) and community composition (Healey and Hovel 4). For other structured marine habitats, however, far less is known about the effects of landscape structure on ecological processes that dictate population dynamics and community structure. Mobile benthic marine species that occupy structured habitats form excellent model systems for the study of how habitat structure at fine scales and landscape scales influence ecological processes (e.g. lobsters: Wahle 1992; Lipcius et al. 1998; Acosta 1999; Butler et al. 5; crabs: Pile et al. 1996; Hovel and Lipcius 1; fishes: Walsh 1985; Levin 1993; Forrester and Steele 4). Spiny lobsters (family Palinuridae) typically occupy discrete, isolated shelters during the day and emerge from shelter at night to forage, and therefore are particularly suitable for studying the effects of benthic landscape structure on behaviour, survival, and population dynamics (e.g. Zimmer-Faust and Spanier 1987; MacDiarmid et al. 1991; Childress and Herrnkind 1997). In this study we focused on the effects of habitat structure on the behaviour and abundance of the California spiny lobster (Panulirus interruptus) in a patchy kelp forest habitat. Little is known about the ecology of P. interruptus relative to many other commercially valuable species such as the American lobster (Homarus americanus) and the Caribbean spiny lobster (Panilurus argus). Spiny lobster females release planktonic larvae that spend c. 1 months in the water column (Mitchell 1971; Phillips and McWilliam 1986) before settling into shallow, structurally complex vegetated habitats (e.g. seagrass, surfgrass, or mangroves) as post-larval pueruli (Engle 1979; Castañeda-Fernández de Lara et al. 5a, 5b). After several years, juveniles shift to a shelter-dwelling phase in rocky subtidal habitats. During this shift, spiny lobster behaviour changes from a solitary existence to a gregarious lifestyle in CSIRO /MF /7/5419

2 4 Marine and Freshwater Research T. T. Mai and K. A. Hovel which juvenile and adult spiny lobsters frequently share shelters (P. interruptus: Engle 1979; Zimmer-Faust and Spanier 1987; Stull 1991; P. argus: Childress and Herrnkind 1997; Jasus edwardsii: Butler IV et al. 1999). At this stage spiny lobsters are shelter-dependent, generally inhabiting spatially discrete shelters during the day singly or in groups and leaving to forage outside the shelter at night (MacDiarmid et al. 1991; Stull 1991). Remaining in shelter during the day (Mintz et al. 1994) and aggregating (Herrnkind et al. 1) may allow lobsters to avoid and deter predators. We presently know very little about the relative influence of habitat structure at the landscape scale on lobster abundance, survival, and movement (but see MacDiarmid et al. 1991; Childress and Herrnkind 1997). Within the California spiny lobster s rocky reef habitat in Southern California, isolated rocky crevices and canopy-forming giant kelp (Macrocystis pyrifera) holdfasts hollowed out by urchins form the majority of the shelters used by juvenile and adult lobsters (T. Mai and K. Hovel, pers. obs.). Isolated stands of understorey algae form another patchy component of the landscape. The goal of the present study was to determine the relative role of habitat characteristics at the scale of individual shelters and at landscape scales on California spiny lobster habitat use and survival. Specifically, we evaluated the relative importance of shelter size and type (local-scale habitat characteristics) v. shelter density, shelter dispersion, and understorey algal density (landscape-scale habitat characteristics) on lobster abundance and size distribution. We made the following predictions: at the individual shelter scale: (i) the number of lobsters per shelter is positively related to shelter size; (ii) because small lobsters may benefit most from aggregation, the mean size of lobsters per shelter is inversely related to shelter size; and (iii) permanent (i.e. rocky) shelters are occupied more frequently than are temporary (i.e. holdfast) shelters. At the landscape scale, we predicted that (i) lobster abundance is positively related to the density of shelters and inversely related to the dispersion of shelters; and (ii) lobster abundance is positively related to, and mean lobster size is inversely related to the proportional cover of understorey algae. Lastly, we predicted that the relative survival of lobsters is higher in the presence of understorey algae than in unvegetated sediment, and is positively related to lobster size. Materials and methods Study area Our study occurred in the Point Loma kelp forest near San Diego, California, a 1-km long 1-km wide area dominated by the surface canopy-forming giant kelp (M. pyrifera) and by understorey stipitate (Pterygophora californica) and prostrate (Laminaria farlowii) kelps (Dayton et al. 1992; fig. 1). The substratum consists of gently sloping sandstone with scattered cobble to boulder-sized rocks of varying densities. Commercial and recreational fishing for lobsters takes place in the kelp forest from October through March. Landscape mapping and lobster habitat use To determine lobster habitat use patterns, in May June 4 we quantified shelter density and distribution in twelve m m California m m 25m Mission Bay Point Loma San Diego Bay North Island 1 km Fig. 1. Map of the coast of San Diego, California showing the Point Loma kelp forest (dotted line). Squares represent the 12 surveyed landscapes. landscapes ( 9 15 m depth) within the kelp forest using SCUBA (Childress and Herrnkind 1997). To facilitate shelter mapping, in each landscape we formed a grid of 36 individually labelled plastic cards affixed to the substrate at 4-m intervals. We then recorded the position of all potential lobster shelters within each landscape (Fig. 2). Lobster shelters were characterised as (i) ledge (cracks in the substrate forming permanent crevices or overhangs), (ii) rock (spaces between rocks or between rocks and the substrate), or (iii) holdfast (large M. pyrifera holdfasts hollowed out by grazing sea urchins or herbivorous gastropods). Lobsters were found using all three shelter types during pilot studies and were rarely seen outside these shelter types during the day (K. Hovel, unpubl. data). Ledge and rock shelters generally are permanent but occasionally are filled with urchins, preventing lobster use. Holdfast shelters decay over time and may be dislodged by wave action, making them more ephemeral. In addition to position, we recorded shelter dimensions (height, width, and depth in cm) to obtain a measure of shelter size. Each dimension was measured at two points and the mean of the two measurements was used for analysis. We quantified stipe densities of the two dominant understorey algae, L. farlowii and P. californica, by randomly placing ten 1-m 2 quadrats in each landscape and counting the number of stipes for each species in July and August 4. Because these two species are long-lived (6 7 years) and the randomised quadrats did not overlap in the two surveys, we combined the July and August surveys for a total of stipe density counts for each landscape.

3 California spiny lobster habitat use Marine and Freshwater Research 421 (a) (c) (b) (d) per landscape was related to shelter density and distance to nearest shelter. The mean distance to the nearest shelter was calculated using the landscape analysis software FRAGSTATS (version 3.: McGarigal et al. 2). To account for collinearity between the predictor variables (shelter density and mean nearest neighbour distance) we regressed mean nearest-neighbour distance on shelter density and used the residuals as a measure of mean nearest-neighbour distance (Zar 1999; Graham 3). Additional multiple linear regressions were used to determine if lobster density and mean lobster CL per landscape were related to P. californica stipe density and L. farlowii stipe density. Collinearity between P. californica stipe density and L. farlowii stipe density was resolved using the above method where the residual of the regression between P. californica stipe density and L. farlowii stipe density was used as a measure of L. farlowii stipe density. Fig. 2. Four example landscapes showing location of shelters within m m areas of the Point Loma kelp forest. (a) Landscape with the highest shelter density; (b) Landscape with the lowest shelter density; (c) Landscape with the greatest shelter dispersion; (d) Landscape with the least shelter dispersion. Circle = ledge shelter, square = rock shelter, triangle = holdfast shelter. Following landscape mapping, we surveyed the landscapes for lobsters during the day when they were relatively inactive within shelters. We recorded the location and size of all lobsters within each landscape once each month for three months (July, August, and September 4) when the kelp forest is closed to commercial and recreational lobster fishing. Lobster size was measured as carapace length (CL) to the nearest.5 cm using a ruler positioned next to each lobster. We analysed data from each monthly survey separately in all tests. At the shelter scale, we used linear regressions to determine if shelter size was related to the number of lobsters per shelter and mean lobster CL per shelter. We used shelter volume (ln transformed) as a measure of shelter size (Zimmer-Faust and Spanier 1987). Results were qualitatively similar using other measures of shelter size in analyses (e.g. height, width, depth, and principle components of these three dimensions). The assumptions of linear regression (normally distributed data, homogeneity of variances and randomly distributed residuals) were assessed graphically and data were ln(x + 1) transformed when appropriate in this and all subsequent regressions. We used a G-test of independence, with shelter type as the independent variable and frequency of lobster presence as the dependent variable, to determine whether lobster shelter use was dependent on shelter type. At the landscape scale, we used multiple linear regressions to determine if the number of lobsters per landscape was related to shelter density and distance to nearest shelter. We also used multiple linear regressions to determine if mean lobster CL Relative survival of lobsters Understorey canopy was found to be an important predictor of lobster abundance (see Results: Lobster density and size v. habitat: landscape scale). Therefore, we conducted a tethering experiment to test the effects of understorey canopy-forming algae on relative survival of lobsters. Tethering is frequently used to compare crustacean relative rates of survival (e.g. crabs: Pile et al. 1996; Hovel and Lipcius 1; lobsters: Eggleston et al. 199; Mintz et al. 1994; Lipcius et al. 1998; Diaz et al. 5). We tethered lobsters ( cm CL) to haphazardly positioned 1-m 2 plots within the kelp forest that either contained understorey algae or did not contain understorey algae. Plots were no less than 5 m from each other and were not placed with the survey plots described above to prevent interference with surveys. The algal treatments consisted of plots that contained only L. farlowii, only P. californica, or both species. Lobsters were captured 2 24 h before being used in experiments and were tethered by tying braided nylon fishing line (36.4 kg test) around the carapace and securing the line to the top of the carapace with cyanoacrylate glue. Tethered lobsters were attached to the substrate using 5 cm masonry nails. Lobsters were exposed to predators for 4 8 days (Eggleston et al. 199) upon which we returned to record the state of the lobster as alive (whole, active lobster remaining on the tether), eaten (fragment of the carapace remaining on the tether), missing (no lobster or fragment remaining on the tether) or molted (entire carapace remaining on the tether). Molted lobsters were excluded from the analysis. In each trial, four lobsters were tethered in predator exclusion cages as controls. Because no lobsters tethered in cages or in laboratory aquaria for up to two weeks escaped from tethers, we considered missing lobsters as being eaten by predators. Lobsters that survived the experiment were immediately released. We tethered 16 lobsters per trial and conducted 5 trials during February and March 5. Some lobsters were lost due to bad weather or poor visibility, and therefore 59 of 96 replicate tethered lobsters could be included in the analysis. We used a multiple logistic regression, with the two understorey algal treatments (understorey algae present or absent) and lobster size (CL) as independent variables and lobster status (live v. eaten) as the dependent variable to determine if lobster survival varied with understorey algal type and lobster size.

4 422 Marine and Freshwater Research T. T. Mai and K. A. Hovel Proportion of shelters with lobsters July August September Table 1. Mean and range of shelter volume for each lobster shelter type in the three surveys, summer 4 Survey Shelter type (n) Mean (cm 3 ) ± s.e. Range (cm 3 ) July Ledge (42) ± Rock (57) 5824 ± Holdfast (23) ± August Ledge (44) ± Rock (57) 5824 ± Holdfast (24) ± September Ledge (47) ± Rock (58) 5846 ± Holdfast (18) ± holdfast shelters and then ledge shelters (Table 1). Rock shelters comprised 46.7%, 45.6%, and 47.2% of all shelters, and ledge shelters comprised 34.4%, 35.2%, and 38.2% of all shelters in July, August and September 4, respectively. Holdfast shelters were least abundant of the three shelter types (18.9%, 19.2%, and 14.6% of all shelters in July, August, and September, respectively). These percentages varied among the surveys due to formation and destruction of shelters (primarily holdfast shelters) or due to changes in occupancy of rock and ledge shelters by urchin aggregations. Shelter density (number per 4 m 2 ) ranged from 5 to 16 for all survey months. Mean nearest neighbour distance, a measure of shelter dispersion, varied from 2.7 m to 11.3 m. Stipe densities for L. farlowii (mean = 6.9 mm 2 ) were consistently higher than for P. californica (mean = 1.1 m 2 ) Lobsters per shelter Fig. 3. Frequency distribution for the number of spiny lobsters found per shelter in July, August, and September surveys in 4. Results Lobster and landscape characteristics We found a total of 131, 151, and 126 lobsters in July, August, and September, respectively (Table 2). Approximately 6% of potential shelters were unoccupied during all of the surveys (Fig. 3). Lobster occupancy of all shelter types combined ranged from to 14 lobsters per shelter, and when shelters were occupied, it was most frequently by a solitary lobster (Fig. 3, Table 2). Shelter volume ranged from 465 to cm 3 with a mean of cm 3 (±1169 s.e.). Mean shelter volume varied with shelter type: rock shelters averaged the smallest, followed by Lobster density and size v. habitat: shelter scale We predicted a positive relationship between the number of lobsters and shelter size, and a negative relationship between mean lobster size (CL) and shelter size. However, we found little evidence for relationships between the number of lobsters per shelter and mean lobster CL, and between the number of lobsters and shelter volume. Linear regressions explained at most 7.3% of the variation in the number of lobsters per shelter and mean CL per shelter (data not shown). Linear regressions between the number of lobsters per shelter or mean CL per shelter, where each shelter type was analysed separately, yielded similar results. In the July and August surveys, however, there was strong evidence that lobsters preferred ledge and rock shelters over holdfast shelters (Fig. 4, Table 3). Across all surveys, percent occupancy of ledge and rock shelters ranged from 4.8% to 55.3%. In July and August 17.4% and 16.7% of holdfast shelters were occupied respectively, but in September the occupancy rate of holdfast shelters more than doubled to 38.9%. Lobster density and size v. habitat: landscape scale We predicted a positive relationship between shelter density and lobster density, and a negative relationship between shelter mean nearest neighbour distance and lobster density. Only the September survey showed a statistically significant positive relationship between shelter density and lobster density (Fig. 5). Additionally, there were no statistically significant relationships between mean lobster CL and shelter density or shelter mean nearest neighbour distance on any sampling date.

5 California spiny lobster habitat use Marine and Freshwater Research 423 Table 2. Total number of lobsters found and mean density and CL for lobsters per landscape (4 m 2 ) and per shelter in July, August, and September surveys, summer 4 Variable July August September Total lobsters Mean lobster density (per 4 m 2 ) 1.92 ± ± ± 1.67 Mean CL (per 4 m 2 ) 5.6 ± ± ±.15 Lobster density (per shelter) ± ± ±.12 Lobster density (per shelter) ± ± ±.17 Mean CL (per shelter) ± ± ±.17 1 Includes all occupied and unoccupied shelters. 2 Includes only occupied shelters. % of shelters occupied Ledge Rock Holdfast Relative survival of lobsters We predicted that lobster relative survival would be higher in the presence of understorey canopy cover than in the absence of algae, and would increase with lobster CL. We found evidence for higher lobster survival in plots with understorey algae v. plots with no algae, but no evidence of a relationship between lobster CL and relative survival (logistic regression: algae: d.f. = 1, χ 2 = 3.5, P =.8; CL: d.f. = 1, χ 2 =.19, P =.66). Lobsters had a 61% probability of survival in the presence of algae and a 48% probability of survival in the absence of algae (Fig. 8). 1 July August September Fig. 4. Proportion of available ledge, rock, and holdfast shelters occupied by spiny lobsters in July, August, and September 4. See text for definition of shelter types. We predicted a positive relationship between understorey algal density and lobster density and a negative relationship between understorey algal density and lobster CL. We found strong evidence for relationships between algal density and lobster density and CL in some months, but relationships were positive or negative depending on the species of understorey algae and the survey month (Figs 6 and 7). In July, lobster density increased as P. californica density increased, and lobster density decreased with increasing L. farlowii density. Mean CL increased as P. californica density increased and no correlation was evident between L. farlowii density and mean CL in July. Lobster density did not vary with either L. farlowii or P. californica densities in August, but as in the July survey, mean CL increased as P. californica density increased and there was no evidence for a relationship between lobster CL and L. farlowii density. The September survey yielded strong evidence for a negative relationship between lobster density and L. farlowii density, but no evidence for a relationship between lobster density and P. californica density, and no evidence of a relationship between lobster CL and density of either algal species. Discussion Spiny lobsters are closely associated with structured habitats (i.e. seagrass meadows, coral reefs, rocky reefs, and mangrove forests) that may offer food and refuge from predation (Engle 1979; Herrnkind et al. 1997; Acosta 1999; Castañeda-Fernández de Lara et al. 5b). The well studied Caribbean spiny lobster, P. argus, prefers shelters with conspecifics and shelters scaled to their body size (Eggleston et al. 199; Eggleston and Lipcius 1992), while at the landscape scale, the type, amount and juxtaposition of habitats influences lobster distribution and abundance (Acosta 1999; Butler et al. 5). While Zimmer- Faust and Spanier (1987) have demonstrated P. interruptus preference for larger shelters and shelters with conspecifics or their scent, there is no information on associations between California spiny lobsters and habitat characteristics at the landscape scale. In our study, we found that lobster abundance and survival varied with features of the habitat at multiple spatial scales, but these responses varied temporally. The density and mean CL of lobsters did not vary with shelter size, but lobsters did prefer structurally permanent shelters (ledge and rock) over temporary shelters (holdfasts) in two of the three surveys. Lobster density and mean lobster size did not vary with shelter density or dispersion, but did vary with the density and type of understorey algae present within the landscape. Moreover, the effects of understorey algae on lobsters appeared to vary through time. When relationships between lobster density and algal density were evident, lobster density was positively related to P. californica density, but negatively related to L. farlowii density. A positive relationship was also evident between P. californica

6 424 Marine and Freshwater Research T. T. Mai and K. A. Hovel Table 3. Presence and absence of lobsters in each shelter type and G-test results of lobster occupancy by shelter type for July, August and September surveys, summer 4 Survey Shelter type Occupancy (number of lobsters) G-score P Present Absent July Holdfast 4 19 Rock Ledge August Holdfast 4 Rock Ledge September Holdfast 7 11 Rock Ledge r 2.114; P.284 July r 2.9; P.771 July r 2.4; P.14 r 2.176; P Lobsters per 4 m 2 1 August r 2.54; P.469 r 2.86; P.355 September r 2.593; P.3 r 2.; P.592 Lobsters per 4 m 2 1 August r 2.166; P.167 r 2.171; P.161 September r 2.45; P.42 r 2.428; P Shelters per 4 m Mean nearest neighbour distance residual Fig. 5. Spiny lobster density (no. lobsters per 4 m 2 ) v. shelter density (no. shelters per 4 m 2, left hand panels), and spiny lobster density v. residual of mean nearest-neighbour shelter distance (right hand panels) for July, August and September surveys, 4. n = 12 for all regressions. density and mean lobster size in two out of three surveys. When lobsters were exposed to predators, the odds of lobster relative survival were higher in the presence of understorey algae than over unvegetated bottom. Overall, our results indicated that lobsters preferred rock and ledge shelters to holdfast shelters, but they did not appear to prefer larger shelters that allow aggregation or landscapes with greater amounts of shelter and closer shelters. Of the variables we studied, the density and type of P. californica per m 2 L. farlowii residual Fig. 6. Spiny lobster density (no. lobsters per 4 m 2 ) v. Pterygophora californica stipe density, and lobster density v. residual of Laminaria farlowii stipe density for July, August and September surveys, 4. n = 12 for all regressions. understorey algae appeared to be the best predictor of lobster density and size at the landscape scale, and algae appeared to offer some protection from predators to lobsters. In our Southern California study area, lobsters appear to base landscape selection more on understorey algal type and abundance than on qualities of shelters. Lobsters and habitat: shelter-scale characteristics We found that larger shelters did not house more lobsters and mean CL did not vary with shelter size. This was, however,

7 California spiny lobster habitat use Marine and Freshwater Research 425 Mean carapace length (cm) r 2.9; P.75 r 2.1; P.65 July r 2.8; P.758 August r 2.86; P.289 September r 2.8; P.393 r 2.29; P P. californica per m 2 L. farlowii residual Fig. 7. Spiny lobster mean carapace length (CL) v. Pterygophora californica stipe density, and mean spiny lobster CL v. residual of Laminaria farlowii stipe density for each survey month. n = 12 for all regressions. No. of lobsters Algae Treatment No algae Eaten Alive Fig. 8. Number of tethered California spiny lobsters eaten or remaining alive after 4 8 days in the presence and absence of understorey algae, in winter 5. contrary to our prediction as well as previous research on this species. Research in mesocosms has demonstrated the relative importance of shelter size to lobster shelter preference, where larger shelters and shelters with other lobsters (or their scent) were preferred (Zimmer-Faust and Spanier 1987). Lobsters likely prefer large shelters because they are gregarious, and larger shelters can house larger aggregations. The group defence hypothesis argues that smaller lobsters that do not possess a size refuge from predation are expected to benefit most from these aggregations, and they therefore should choose shelters that maximise lobster density and hence their survival (Mintz et al. 1994; Childress and Herrnkind 1997). However, there is the potential for gregariousness to facilitate predation by concentrating prey (Lavalli and Spanier 1; C. Loflen, unpubl. data). Alternatively, P. argus preferred shelters scaled to their body size rather than larger shelters, which may confer greater protection from predators (Eggleston et al. 199). The lack of relationship between shelter size and lobster density, and shelter size and mean lobster size in our study may be a product of relatively low predation risk for lobsters and relatively low lobster density in the Point Loma kelp forest. Though our study demonstrated that lobsters experience some predation risk, and lobsters exhibit behaviours (such as daytime shelter use) that likely have been selected for by predation, densities of large piscine predators (e.g. giant seabass, lingcod and California sheephead) in the Pt. Loma and nearby La Jolla kelp forest are lower than historic values due to fishing (T. Mai and K. Hovel, pers. obs.; P. Dayton, pers. comm.). Group defence and other aspects of aggregation that may lower predator-induced mortality therefore may not be selected for under present conditions, and lobsters at this location may instead be responding to different aspects of their habitat (e.g. food levels) or to habitat characteristics at a different spatial scale. Additionally, shelters likely are not limiting for lobsters due to relatively low lobster densities, and lobsters therefore are not being forced to aggregate due to low shelter availability. Moreover, it is possible that low densities inhibit lobsters from finding enough conspecifics to form large aggregations, though spiny lobsters are able to accelerate shelter acquisition by detecting olfactory cues from conspecifics (Zimmer-Faust and Spanier 1987; Childress and Herrnkind 1997). Finally, there are alternative habitats for California spiny lobsters in our study area, such as surfgrass (Phyllospadix spp.), in which we have seen large lobster aggregations. Future studies on lobster habitat use should consider these alternative habitats. Our results suggest that shelter type drives shelter selection more so than shelter size. In this rocky reef system, shelter types include ledge shelters, rocky crevices, and hollowed-out kelp holdfasts. Lobsters may have preferred ledge and rock shelters over holdfast shelters because holdfast shelters are more ephemeral. More permanent ledge and rock shelters are more conducive for homing behaviour (the repeated use of a shelter) exhibited by this species (Stull 1991; K. Hovel, unpubl. data) and other Palinurid species such as Jasus edwardsii (MacDiarmid et al. 1991). Homing may facilitate location of shelter following nightly foraging excursions. In addition, these shelters may provide better and sturdier cover from predators than holdfast shelters, which simply are cavities within M. pyrifera holdfasts that may be easily dislodged by wave-action or drifting kelp masses. Interestingly, there was little difference in rate of occupancy among shelter types in September, when holdfast shelter percent occupancy increased dramatically (Table 3). This was a result of a 25% loss of holdfast shelters between the August and September surveys and an increase in occupancy of the remaining holdfast shelters.

8 426 Marine and Freshwater Research T. T. Mai and K. A. Hovel Lobsters and habitat: landscape-scale characteristics California spiny lobsters travel tens to hundreds of metres during their nightly foraging excursions from shelters (Stull 1991; K. Hovel, unpubl. data). Homing behaviour may not only occur at the shelter scale (i.e. to particular shelters), but also at the landscape scale (e.g. particular landscapes can offer more resources: MacDiarmid et al. 1991; Kelly and MacDiarmid 3). For instance, in New Zealand, J. edwardsii travelled a median distance of less than 5 m during nightly foraging excursions and tended to return to the same shelter or nearby shelters primarily when shelter availability was high and when landscapes contained dense stands of the algae Ecklonia radiata, suggesting that these lobsters prefer landscapes with many shelters and dense understorey algae (MacDiarmid et al. 1991). Landscapes with high shelter density may be able to house more lobsters and offer a greater choice of shelters for lobsters. We therefore expected a positive relationship between shelter density and lobster density in all surveys, but this occurred only in September. September represents a seasonal shift in behaviour where a subset of the spiny lobster population, mainly larger reproductive adults, makes a seasonal migration to greater depths in the kelp forest (Mitchell 1971), which could lead to higher shelter availability for remaining lobsters or behavioural changes due to altered dominance hierarchies. We also expected an inverse relationship between shelter dispersion and lobster density. Landscapes with low shelter dispersion may allow lobsters to form larger groups among nearby shelters and may facilitate the location of shelter following nightly foraging excursions. However, P. interruptus did not exhibit a preference for landscapes with low shelter dispersion in any of the three surveys. These results may have been the product of low lobster densities relative to the availability of shelters and low rates of shelter occupancy. In all three surveys the mean proportion of unoccupied shelters per landscape ranged from.59 to.63. Thus, there may be little competition for shelters among lobsters, even in landscapes with low shelter densities. Additionally, Kelly and MacDiarmid (3) found that female J. edwardsii were more likely to associate with particular sites or landscapes with increasing lobster size, and that males over 13. cm carapace length had higher site fidelity. Lobsters in our study were much smaller (mean CL = 6. cm) and therefore may have been less likely to be selective of landscapes. Understorey kelps are prominent components of temperate rocky reef systems such as the Point Loma kelp forest. Understorey kelp density influences recruitment of fishes (Carr 1989; Levin 1993) and sessile invertebrates, such as polychaetes, bryozoans and bivalves (Duggins et al. 199; Connell 3), and distribution of urchins (Konar ). Association between understorey kelp and P. interruptus has not been studied (but for H. americanus see Bologna and Steneck 1993); however, California spiny lobsters use other types of vegetative structures (e.g. eelgrass Zostera marina and surfgrass Phyllospadix spp.: K. Hovel, unpubl. data) as does P. argus (Sosa-Cordero et al. 1998; Acosta 1999). Panulirus argus uses seagrass meadows as foraging habitat (Sosa-Cordero et al. 1998) and as refuge corridors between habitats (Acosta 1999). It is conceivable that understorey kelps function in such a manner for P. interruptus. A tracking study using radio-tagged California spiny lobsters in our study sites following our surveys revealed that lobsters followed corridors of understorey kelp to cross open areas between the kelp forest and surfgrass habitat in nocturnal foraging movements (K. Hovel, unpubl. data). Moreover, our tethering experiment indicated that lobster relative survival is higher in the presence of understorey kelp than over bare substratum. While this suggests that lobster density should be positively related with algal density, we found a positive relationship between lobster density and P. californica density in July but not in August or September. The relationship between P. californica density and lobster density may have been more evident if our surveyed landscapes had covered a broader range of P. californica densities, including more areas devoid of understorey algal growth. In two of our surveys we found a positive relationship between P. californica density and lobster size. This result is counter to the argument that larger lobsters rely more on a size refuge v. a habitat refuge. However, most of the lobsters we found were juveniles or sub-adults and likely do not yet possess a size refuge from predation. In our tethering experiment we used lobsters of similar size (mean CL = 5.4 ±.15 cm (s.e.)) and found that lobsters within this size range were all susceptible to predation. It is also possible that larger lobsters have learned to select areas of dense understorey kelp through which to travel, while the smaller lobsters have not yet learned this behaviour or prefer other vegetative habitats such as surfgrass (Engle 1979). In contrast to P. californica, lobster density was negatively related to L. farlowii density in two of three surveys, and there was no relationship between L. farlowii density and lobster size. Laminaria farlowii has a different morphology than P. californica. Pterygophora californica has a rigid stipe with numerous blades at the top (resembling a palm tree), while L. farlowii has a short thin stipe with a single large blade. L. farlowii has a tendency to sweep or whip the substrate as it is pushed back and forth by wave-action. Similar whipping motion in Desmarestia viridis limited sea urchin distribution in the Aleutian Archipelago (Konar ). Additionally, recruitment of sessile invertebrates in Southern Australia was reduced with increasing physical abrasion by algae (Connell 3). While it is unlikely that abrasion by kelp can damage lobsters to the same extent as sessile invertebrates, abrasion can create a stressful environment that deters the use of patches of L. farlowii, or, may prevent colonisation by organisms eaten by lobsters. Future research directions Limited research has been conducted on the ecology of P. interruptus. Our research emphasised the role of habitat at multiple spatial scales on lobster abundance and distribution. We suggest expanding this approach and focusing on other locations where environmental conditions and lobster densities may be different from the Point Loma kelp forest. For instance, areas of higher lobster densities (e.g. Baja California) may result in shelter limitation leading to the greater influence of shelter size, density or dispersion on lobster distribution. Areas with higher predation risks (e.g. marine protected areas) may alter the relationship between habitat characteristics and lobster habitat use, as predation can influence gregariousness, homing behaviour or movement patterns. Additionally, future studies in this system should include vegetative characteristics that have been shown

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