Transplanting of wild and cultivated juveniles of green abalone (Haliotis fulgens Philippi 1845): growth and survival.

ABSTRACT The growth and survival rate of wild and cultivated abalone juveniles (Haliotis fulgens) transplanted into the sea was measured from January 2000 to January 2003 in Bahia Tortugas, Baja California. A total of 306 wild individuals (17-90 mm shell length) and 529 cultivated juveniles (16-49 mm shell length), were tagged and released in the intertidal zone. Six surveys were carried out during the experiment. The recaptured organisms were released, each time, in the same place once the shell length was recorded. Growth data were fitted to Gompertz growth model. Mortality rate was estimated through an exponential decay model. The growth rate 1 year after release was 52.4 mm/year for wild abalone and 48.1 mm/year for cultivated abalone. In the third year these rates decreased to 30 and 28 mm/year respectively. No significant difference was found. Gompertz equation parameters were very similar for wild ([L.sub.0] = 2.34, G = 4.30; g = 0.827) and cultivated abalone ([L.sub.0] = 2.14, G = 4.38; g = 0.812). Natural mortality rates were M = 1.20 [y.sup.-1] for cultivated juveniles and M = 0.87 [y.sup.-1] for wild juveniles. No significant difference was found between both. Sighting efficiency rate was: 74% for both types of juveniles. This proportion is indicative that wild and cultivated juveniles had a similar dispersal and cryptic ability. We conclude that annual survival rates: 30% for cultivated juveniles and 42% for wild juveniles could be sub estimated.

KEY WORDS: growth, survival, transplanting, juveniles, Baja California, Haliotis fulgens

INTRODUCTION

Fishing of abalone (Haliotis spp.) is one of the leading economic activities in the regional economy of Baja California. Native populations of this mollusk have undergone a sharp decline in numbers since the 1970s, as a result of increased fishing pressure as well as possible climate changes (Guzman del Proo 1992, 1994). Commercial landings and biomass are estimated to have declined in some areas to one-tenth of their original numbers, in comparison to the 1950s (Guzman del Proo 1992, 1997).

The outplanting of hatchery-reared abalone for the purpose of restocking natural reefs has been practiced routinely in Japan (Kojima 1995, Seki & Tanaguchi 2002). Recently, successful seedings experiments have been conducted in Australia (Dixon et al. in press). These countries have also been interested in promoting the sale of cultivated abalone in international markets.

In Mexico, the fishing cooperatives of Baja California maintain abalone hatcheries for the specific purpose of restocking their reefs. Similarly, they occasionally transplant wild juveniles from "stunted stock" areas to banks known for their "good growth." To date, however, both practices have lacked the follow-up that might allow assessment of the survival of the seeded stock as well as of its impact on the total biomass of the wild populations. Although data exists on the growth of adult Haliotis fulgens Philippi 1845 and Haliotis corrugata Gray 1828 (Marfn 1981, Turrubiates 1989, Shepherd et al. 1991, Guzman del Proo & Lopez-Salas 1993), little is known of the juvenile stages of these species.

The purpose of this study is to determine the growth rate and survival of wild and cultivated abalone juveniles following their release at sea, to evaluate the impact of enhancement programs on wild populations of these mollusks.

STUDY AREA

The study was done in the rocky intertidal zone of "Varadero" at Clam Bay (27[degrees]37'11"N and 114[degrees]50'38"W), a small cove located southeast of Bahia Tortugas (Fig. 1). The study site is a low-relief shelf with many imbricate rocks sheltered from the dominant northwest winds. Juveniles of H. fulgens are abundant in this location.

[FIGURE 1 OMITTED]

METHODS

A total of 835 juveniles of green abalone (Haliotis fulgens), 529 cultivated and 306 wild, were tagged. Cultivated juveniles ranged in size from 16 to 49 mm length, with a mean of 23.5 mm, and were 1 year old. Wild juveniles had a broader size range, from 17 to 90 mm (mean 53.5 mm), which is approximately equivalent to 1 to 2 years of age.

The wild specimens were captured at sites near Bahia Tortugas and taken to the laboratory in ice chests containing water and giant kelp (Macrocystis pyrifera (L.) Agardh 1820). They were measured and tagged along with the cultivated individuals that were obtained from the Aquaculture Unit of Cooperativa Bahia Tortugas. All specimens were allowed to rest for 24 hours in tanks with running seawater to cull damaged individuals.

Numbered polyethylene ribbon tags (Hallprint, South Australia) were fixed to the shells with cyanoacrylate glue (Three Bond TB-1747). Specimen length was measured to the nearest millimeter with vernier calipers.

A random mix of wild and cultivated juveniles was released at low tide in January 2000. In one case, the specimens were attached by hand to the underside or side of boulders, 30 x 60 cm on average, covered in the upper surface by calcareous or foliose algae, but with smooth undersides. Juveniles were planted at densities of 4 to 7 per boulder, according to the microhabitat observations of Carreon-Palau et al. (2003). In another case, 25 to 30 juveniles were placed in 30-cm long, 4-cm diameter PVC tubes in the laboratory, then taken to sea and deposited in cryptic locations among the rocks to allow the specimens to abandon the tubes gradually and choose suitable sites.

The release area was marked off by two 25-m long steel chains placed at right angles to the shoreline and separated from each other by a distance of 75 m. Each chain was also marked at 5 m intervals with small plastic chains as reference points. The seeding and recapturing area was approximately 200 [m.sup.2].

Recaptures were undertaken in summer and winter months, six in total, from June 2000 to January 2003, recording the size and tag number of each specimen. Searching was done always on days when the tide was at its lowest, starting with the release sites and their surrounding area and applying a constant search effort of six people for 1 hour. However, because summer ebb tides (June to July) were not enough to leave the boulders totally exposed, and in addition, a heavy Macrocystis canopy is characteristic of this time of the year, the searching efficiency was seriously affected. Thus, we decided to use for the final analysis only the winter data surveys, when we found the lowest tides and less densities of macroalgae cover.

Most recaptured juveniles were measured in situ. When this was not practicable, the specimens were carefully detached to avoid excessive handling. After measurement, they were replaced on the rock, making sure they were attached to it.

The data of the mean length by age of recaptured specimens, both wild and cultivated, were fitted to Gompertz growth model (Rawlings 1988, Ricker 1975).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]

Where:

Lo = the initial SL at time [t.sub.0].

G = the instantaneous growth rate at time [t.sub.0], and

g = describes the rate of decrease of G.

Both, cultivated and wild mean length-increments, were compared by ANCOVA test (Schefler 1980).

To estimate survival, the raw data for the total number of wild and cultivated specimens recaptured each year was corrected as follows: tagged specimens not sighted before the second and/or third recapture were considered to be survivors in the previous periods. This correction allowed us to obtain a more accurate estimate of minimum survival.

The proportion of individuals not sighted before, but sighted in subsequent surveys was used to estimate the sighting efficiency (Dixon et al. in press). An Arcsine transformation (Krebs 1999) was used to normalize proportions prior to calculate sighting efficiency.

The corrected abalone counts were fitted to an exponential decay model (Rawlings 1988, Gulland 1975) to estimate the instantaneous rate of natural mortality (M) after seeding, from January 2001 to January 2003. An ANCOVA analysis was run to compare statistical significance of the wild and cultivated slopes (Schefler 1980).

RESULTS

To be able to compare growth and survival estimates in both populations, in the case of wild abalone only specimens whose initial size range was between 17 and 34 mm SL (n = 112; mean length = 24.8 [+ or -] 0.87 mm) were considered in the analysis. We assumed this range included individuals approximately 1 year old, as in the case of the cultivated specimens which size ranged between 16 to 34 mm length (n = 520; mean length 24 [+ or -] 0.33 mm). Figure 2 indicates the size-frequency distributions of the wild and cultivated specimens used for the analysis.

[FIGURE 2 OMITTED]

Growth

Figure 3 shows the mean annual growth of wild and cultivated juveniles of H. fulgens. In general, both groups attained 147 [+ or -] 8 mm in length 3 years after release. Because juveniles were released when they were 1 year old, at 4 years both populations were virtually ready to recruit to the fishery, which has a commercial minimum size of 145 mm for H. fulgens (Ortiz-Quintanilla & Carballo 1988).

[FIGURE 3 OMITTED]

The growth rates observed in the first year following release (at age 2 y) were 52.4 mm/year in wild juveniles and 48.1 mm/year in cultivated juveniles. These rates decreased to 29.8 and 28.1 mm/ year, respectively, in the third year following release (at age 4 y) (Fig. 4). No significant difference was found between the average growth of the wild and cultured abalones ([F.sub.(0.05,1,3)] = 0.3633, P < 0.0001).

[FIGURE 4 OMITTED]

The following equations indicate the fit of the data to the Gompertz growth model

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] for wild abalone

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] for cultivated abalone

Both equations look similar and the small differences observed are not significant, as ANCOVA showed in growth rates comparison (Fig. 4).

Survival

Taking into account the total number of recaptured specimens over the 3 years of the experiment, 81.6% on average were recaptured once (81.8% wild and 81% cultivated), 17.2% twice (13.6% and 19%, respectively) and only 1.1% of specimens were recaptured on all three occasions (4.5% and 0%, respectively).

The number of tagged empty shells recaptured over the course of the study was very small. Tag-induced mortality or stress involved only 2 organisms 24 hours after tagging. Six months later two other empty shells were found, but these were attributed to predation.

All the recaptures of tagged specimens were found in cryptic locations. In general, specimens showed limited dispersal over the first year, the maximum magnitude of movement with respect to the release area being 2 to 4 m. Dispersal rose slightly in the second year, from 4 to 7 m, whereas by the third year, some specimens were found to have migrated up to 15 m beyond the site of their initial release. At 1.5 years after the start of the experiment, three specimens turned up in commercial catch about 50 m away from their release sites, at 3 m or 4 m depth. This suggests that as the experiment proceeded some specimens moved gradually toward deeper water. Because the search area was restricted to the lowest tide line, individuals that may have moved into the subtidal zone, like those turning up in commercial catch, would not have been recorded.

Table 1 shows the total number of recaptured abalone, including sighted and not sighted ones, after seeding in January 2000. Based on these figures the estimated values of sighting efficiency rate were: 74.2% for cultivated and 74.3% for wild juveniles.

The rates of natural mortality, calculated after seeding, from January 2001 to January 2003, were M = 1.20 [y.sup.-1] for cultivated juveniles and M = 0.87 [y.sup.-1] for wild juveniles (Fig. 5). ANCOVA test indicated no significant difference between the 2 mortality rates ([F.sub.(0.05,1,3)] = 0.0185, P < 0.0001). Mortality values converted to annual survival rates were S = 30% for cultivated juveniles and S = 42% for wild juveniles.

[FIGURE 5 OMITTED]

DISCUSSION

Growth

The mean increments observed 1 year after release (48 mm in cultivated specimens and 53 mm in wild ones) indicate a high rate of growth for this species. In exceptional cases, some specimens showed increments higher than 90 mm after the first year following release.

These values exceed previous growth estimates (35 mm/year) reported for 2-year-old H. fulgens juveniles in the area of Bahia Tortugas (Turrubiates 1989). In experiments involving the tagging of individuals larger than 80 mm, Shepherd et al. (1991) found increments ranging from 10-40 mm/year. These growth rates are similar to those reported by Guzman del Proo and Lopez-Salas (1993) for this same species in Bahia Asuncion: 10 to 45 mm/year for abalone larger than 70 mm in length.

The rapid growth observed for both cultivated and wild specimens in the Clam Bay area may be explained by the fact that this site is an optimal habitat for this species, offering a broad availability and abundance of suitable food algae. On the other hand, the intertidal shelf where the experiment took place is of low relief and has many overlapping rocks favoring entrapment of Macrocystis and other drift algae which are easily transported to the hollows and crevices formed by the juxtaposition of rocks in this area (Carreon-Palau et al., 2003). Japanese workers have observed large clusters of Haliotis at depths where drift algae are abundant, and high densities of juveniles in shallow habitats with pebbles (Inoue 1973, Kojima 1974, cited by Seki & Taniguchi 2000).

The observed growth rates, along with the existing data on maturity and fecundity of this species (Guzman del Proo 1992, Gluyas-Millan & Talavera-Maya 2003), allow us to estimate that recruitment to the adult stock could take place at age 4 years, (i.e., when specimens attain a mean SL of 145 mm). Based on this, the 1-year-old juveniles currently being seeded on natural reefs may be expected to enter the fishery 3 years later. However, it should be pointed out that it would be safer for the fishery, to capture abalone equal to or larger than 160 mm SL (Shepherd et al. 1991), because although H. fulgens shows the first maturity signs since 105 to 115 mm (Garcia Juarez & Ortiz Quintanilla 1992, Guzman del Proo et al. 1980, Martinez 2003), catching H. fulgens bigger than 145 mm will be assuring several spawning events have taken place and with higher fecundity levels too (Martinez 2003).

Survival

The mortality values obtained in this study of 0.87 [y.sup.-1] for wild juveniles and 1.2 [y.sup.-1] for cultivated abalone fall well within the range from 0.2 to 4.6 for different species, reported by Shepherd (1998). Because we do not have accurate measurements of the proportion of the seeds that moved out of the area, these values might he better considered as a disappearance rate, which includes mortality and migration.

The equivalent annual survival rates for these values: 30% for cultivated abalone seeds and 42% for wild abalone, represent the minimum survival estimates and is likely to be an underestimate because, as we said earlier, we neither have data of juveniles moved away nor enough extra data as empty shell or broken shell collections of the reef that could adjust our estimates. This scarcity of empty or broken shell collections during the surveys could be explained by our 6-month-apart surveys (shells mean half-life is 48 days, as occur in other species like H. laevigata [Shepherd 1998]), which made it difficult to find them.

The survival estimates obtained in these types of tag-recapture experiments tend to be biased, because they are affected by diverse factors: some mortality due to the deleterious effect of the type of tag, disturbance as a result of handling during seeding and recapture, and the fact that abalone tend to disperse when disturbed (Day & Fleming 1992, Officer et al. 2001). A further source of bias is the error as a result of low effectiveness in the search of tagged specimens (Shepherd & Breen 1992).

In our experiment, the negative effect of the tags was kept to a minimum by fixing them to the shell without affecting the respiratory pores or other structures. Nevertheless, despite the small number of empty shells recaptured (only 2), we do not rule out the possibility of some mortality being induced by seeding and subsequent recapture.

One of the major concerns was the effectiveness of searches. In our case, the sighting efficiency may have been diminished by the difficulty of turning over large-sized rocks. Despite the fact that the juveniles were seeded on movable rocks, the search had to be extended to the largest possible number of neighboring rocks, including some which were difficult to move. On the other hand, despite the care taken in moving the rocks, each search operation probably caused some dispersal of tagged specimens and an indeterminate number of them may have escaped our sight within the release area.

Another factor was the algal cover, mainly Macrocystis canopy, which was very thick at times and made thorough searching in certain areas difficult, particularly in summer. Hence, for the final survival estimates we have used only the annual data for each winter.

A salient fact in the experiment was the vast capacity for dispersal and crypsis shown by both wild and cultivated juveniles. It is well known that juvenile abalone are highly mobile and cryptic (Roger-Bennet & Pearce 1998). At the start of the experiment, most specimens were sighted near the release site, but as time went on it was observed that some of them (Shepherd et al. 1991) and other species moved into deeper water (Tart 1995).

The fact that 3 adult-sized specimens (2 cultivated and 1 wild) turned up in commercial catch (1.5 years after released) indicates that some tagged specimens moved beyond the original seeding area. Using this data to get some value of the migration rate, we estimate 0.4% for cultivated and 0.8% for wild abalones. However, these proportions hardly could be considered representative of the real migration rate values, because the specimens appeared only by chance in the commercial catch and not after an addressed survey.

Laboratory experiments have shown that wild juveniles are better able to find cryptic shelter and avoid predation than cultivated juveniles (Tegner & Butler 1989, Schiel & Weldon 1987 cited by Roger-Bennet & Pearce 1998). We did not find this difference. The sighting proportion showed almost identical values (74.2% and 74.3%) for both kind of juveniles.

We suppose that by extending the experiment period to 3 years, the probability of dispersal increased and the likelihood of sighting an indeterminate number of specimens therefore declined. Thus, the survival values reported in this study may actually be underestimates and suggest the need to perform further experiments with a different experimental design to estimate more accurately how many disperse out of the search area.

One of the major concerns of juvenile abalone seeding programs that aim to enhance the numbers on natural reefs is to increase survival and minimize production cost (Tegner & Butler 1985). Efforts to rebuild depleted or declining abalone stocks have been a concern of abalone-producing countries for several years and have furthered work on the transplanting of laboratory juveniles in Japan (Kojima 1995), the United States, (Rogers-Bennett & Pearce 1998), New Zealand (Sweijd et al. 1998), and Australia (Dixon et al. in press). In some cases, there have been transplants with relative success (Kojima 1995, Dixon et al. in press, Schiel 1993) but nevertheless, most attempts have had poor results to date (Withler 2000, Tegner & Butler 1985).

In our case, even admitting the possibility of error due to underestimation, our results do not hold out great expectations for programs aimed at recovering currently depleted stocks. We believe that at least hundreds of thousands of seeds, more than 25 mm, would have to be planted to maximize survival by seeding more robust individuals (Sweijd et al. 1998, Inoue 1976, cited by Kojima 1995). This action could ensure that a greater number of survivors impact on the production in later generations leading to an increase in biomass; however, the production cost rise with age at release (Sweijd et al. 1998). In addition, we must consider the effects of the sudden introduction of a large number of juveniles, which may not necessarily result in increased biomass, because these specimens may be subjected to competition or predation through density-dependent predation (Hilborn 1998, cited by Shepherd et al. 2000). In other instances, they may produce an aggregative numerical response in predators (Dajoz 1996, Smith 1996), because of the number of prey, in this case the seeded juveniles, increases (Shepherd et al. 2000).

On the other hand, as long as the genetic structure of Haliotis populations and how it is affected by massive introductions of hatchery-reared juveniles is not well understood (Withler 2000), and whether the seed will promote recovery of stocks, it will be difficult to achieve success in attempts to enhance or recover depleted stocks. However, this issue is really beyond the scope of this study.

Therefore, at the present time, it seems that in view of the cost, and uncertainty about the benefits offered by seeding, it is wiser to manage existing populations sustainably. This involves an understanding of metapopulation structure of exploited population and requires minimum adult densities in the region. About this, Shepherd and Partington (1995) suggest for Australian waters minimum densities of 0.2-0.3 adults/[m.sup.2] in source habitats, but we still should determine our own figures.

TABLE 1.

                      Cultivated Juveniles

 Survey                   Not                Sighting
  Date       Sighted    Sighted    Total    Proportion

Jan. 2001      48         19        67         0.72
Jan. 2002      25          5        30         0.83
Jan. 2003       4          2         6         0.67

                         Wild Juveniles

 Survey                   Not                Sighting
  Date       Sighted    Sighted    Total    Proportion

Jan. 2001      15          8        23         0.65
Jan. 2002       9          2        11         0.82
Jan. 2003       3          1         4         0.75

ACKNOWLEDGMENTS

The authors thank the aqua culturist technicians of the Sociedad Cooperativa de Produccion Pesquera Bahia Tortugas for their support with field work and the Centro Regional de Investigacion Pesquera de La Paz who allowed us to use their facilities in Bahia Tortugas Laboratory. The authors also thank Rob Day and Scoresby Shepherd for their helpful critical review that improved the manuscript. This work received financial support by Instituto Politecnico Nacional through the project grants CGEPI 200494 and CGEPI 20020627.

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SERGIO A. GUZMAN DEL PROO, (1,3,*) JORGE CARRILLO-LAGUNA, (1,3) JORGE BELMAR-PEREZ, (1) LAURA CARREON-PALAU (1) AND ALBERTO CASTRO (2)

(*) Corresponding author. E-mail: [e-mail]

(1) Escuela Nacional de Ciencias Biologicas, I.P.N. Prol. Carpio y Plan de Ayala S/N, Mexico, D.F. 11340; (2) Sociedad Cooperativa de Produccion Pesquera, "Bahia Tortugas" Bahia Tortugas, B.C.S.; (3) Becario COFAA