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Figure 1. Fishery location: Sandalwood Peninsula, Western Australia. [Figure produced by Maptool, a product of SEATURTLE.ORG. (www.seaturtle.org)]

Fishing operations were conducted from a base camp on Sandalwood Peninsula, Exmouth Gulf, Western Australia (Fig. 1). This is an arid sub-tropical coast with median precipitation of <250 mm per annum. Rainfall is highly variable from year to year, with tropical cyclones crossing the coast in the summer-autumn period generally bringing the heaviest rains. The driest months are September through December. Freshwater and sediment discharges from short local water courses follow only major rainfall events. At other times outflow from the land is negligible. Mean ambient water temperatures in Exmouth Gulf range generally from ~30°C in the summer (January to March) to ~17°C during the winter months (June to August), but these limits can be exceeded by 1-2°C either way in the shallows (Fig. 2). Air temperatures vary more, with extreme summer maxima exceeding 40°C, and lowest winter minima below 10°C (Australia, Bureau of Meteorology Climate Data Online 2010). Open pan evaporation potential is >3000 mm per annum (Atlas of Australian Resources 1986).

Table 1. Turtle species in the major juvenile and adult size class categories recorded associated with a nearshore commercial net fishery at Sandalwood Peninsula, Exmouth Gulf, Western Australia (based on our then known adult female minimum CCLs for these populations: [n =number of turtles in group (% =of species group sample)].

Figure 2. The hourly temperature records for the years 1998 (dark line) and 1999 (grey line) taken from the bottom at ~6m in Gales Bay, Exmouth Gulf, Western Australia: Station at 22° 26.25’S; 114° 10.75’E. Unpublished data by permission: Anthony Hart, Fisheries Department, WA.

A semi-diurnal tidal regime (range of ~2.8 m maximum) over a wide inter-tidal zone facilitated operation of a fixed net and trap (FAO 2006-2012; gear type 246 - the Japan ‘set net’). This gear extended outward ~120 m from the high water tide level onshore, and comprised a length of fine mesh (21/8” or 31/4” [~54 mm or 82 mm] mesh) net extended from the shoreline across the intertidal zone to a point where a corral was formed to hold fish being deflected from their usual longshore travel by the wing-net. These mesh sizes did not entangle turtles. However, the impediment imposed by this set net could also lead to juvenile turtles (mainly) becoming stranded on the falling tide on the reef flat or in the shallow pools therein, or sometimes captured with the fish. Fish were collected and dealt with as first priority. Any turtles in proximity to the net or within the trap were collected later for recording, tag and release. Additional turtles discovered stranded at other times and places were included when practicable.

Fish foraging on the flood tide within the mangrove community fringing other parts of the Sandalwood Peninsula shoreline could also be taken by seine net (FAO 2006-2012; gear type 102). Nets of maximum 500 m length (2⅛” or 3¼” [~54 mm or 82 mm] mesh) were deployed when fishing, being run out by boat from the shore through a gap in the mangroves, along outside the mangrove frontage, and thence back on shore through another gap. Fish and any turtles also foraging within the mangroves were thus retained within the net barrier on the falling tide, and could be dealt with as those captured via the trap. Hand jigging for squid in season did not catch any turtles.

This simple cooperative arrangement resulted in the recording, tag and release of nearly 1000 turtles between December 1990 and June 1998. Data obtained are discussed below.

Turtle species. Between December 1990 and June 1998, 998 turtles of three species: viz, 882 green turtles (88.4%); 92 loggerhead turtles (9.2%); and 24 hawksbill turtles (2.4%) were recorded, tagged and released. Flatback and leatherback turtles are known from trawl fishery captures within the northern parts of Exmouth Gulf (WAMT Project unpublished data), but none were found within the Sandalwood Point fishery. Olive ridley turtles have not been recorded in the Exmouth Gulf region (Prince et al. 2010).

Species composition. Turtles generally were not assigned to sex in this work due to the usual technical difficulty of doing that without internal examination, and the fact that nearly all the turtles were being acquired and recorded as an adjunct to a commercial fishery by the fishers. The only exceptions were turtles with the diagnostic long tails that could confidently be classed as male, or short-tailed turtles known to have visited a nesting beach elsewhere that could be classed as female (these turtles carried flipper tags). All other turtles were initially recorded as indeterminate sex.

Noting the above, extensive work on Western Australian nesting beaches (WAMT Project unpublished data) has indicated minimum sizes of mature females by species. Thus, the absolute smallest Western Australian nesting green and loggerhead females observed laying eggs were of similar midline curved carapace length (CCL): viz, ~83 cm; and the smallest nesting hawksbills were ~70 cm. Using these size criteria, the majority of the green and loggerhead turtles captured and recorded were likely juveniles, and in similar proportion of juvenile to possible adult turtles (3J:1A); the hawksbill turtle sample was evenly split on both counts (Table 1). These data were part of the WAMT Project information previously contributed to the consideration of the formal export certification for US markets access of produce from the Western Australian Exmouth Gulf prawn (shrimp) trawl fishery. However, the size class distribution of the turtles within each species group differed (Fig. 3).

Figure 3. Comparison of size class distributions by sea turtle species recorded in association with a nearshore commercial net fishery at Sandalwood Peninsula, Exmouth Gulf, Western Australia.

The maximum size class offset between the hawksbill and green turtle groups (Fig. 3; largest hawksbills <100 cm; largest greens >100 cm) is consistent with the western Australian regional populations of hawksbill turtles that are physically smaller animals at maturity than the green turtles. Similarity between the size range spread observed for each of these species suggests overlap in patterns of recruitment to and potential occupancy of the habitats sampled by this mixed Exmouth Gulf fishery. The green turtle sample is however dominated by turtles of the 40 - 49.9 cm CCL size class (~54% of total), with a reasonably even spread (~3 - 7% per band) of turtles across all other 10 cm CCL bands above this, except for a slight excess (~15%) of turtles within the 90 - 99.9 cm band (Fig. 3). This size band includes the average CCL (~98 cm ± 0.8 SE) of nesting adult female green turtles recorded across all Western Australian nesting groups. The hawksbill turtle sample likewise shows a reasonably even spread of turtles across the other 10 cm CCL bands above the minimum, but with an excess (~33%) of adult sized turtles in the 80 - 89.9 cm band (Fig. 3). Again, this latter band includes the average CCL (~86 - 88 cm) of nesting adult female hawksbill turtles recorded across all Western Australian nesting groups.

The loggerhead turtle sample is consistent with expected recruitment of loggerheads to these inshore neritic habitats as advanced juveniles. The majority (~62%) of the loggerhead turtle sample was within the large sub-adult to minimum size adult band of 70 - 79.9 cm CCL. Most others were potential adult sized turtles spread equally across the 80 - 89.9 cm (~15%) and 90 - 99.9 cm (~16%) CCL bands (Fig. 3). The average CCL (~96 cm ± 1 SE) of nesting adult female loggerhead turtles recorded across all Western Australian nesting rookeries falls within the upper of these two bands.

Table 2. Modes of association by sea turtle species recorded interacting with a nearshore commercial net fishery at Sandalwood Peninsula, Exmouth Gulf, Western Australia. *A two-handed drag net of 10 m length by ~180 cm depth of ~150 mm square mesh mounted between two poles was tested initially for this project, and proved effective for targeted capture of juvenile turtles from among mangroves, but was labor intensive (see other data below), and not continued.

It is apparent from Table 2 that there were differences between species in their associations with the netting operations and strandings at low tide. These results suggest an underlying pattern of differences between the three species in habitat occupancy and usage, resulting in different patterns of interactions with the particular fishing operations and related local environmental factors. A breakdown of Table 2 data into 10 cm band size classes of turtles plus the modes of association with the fishery for the loggerhead (Table 3) and green turtles (Table 4) show further differences between the species.

Table 3. Interaction modes for the different size classes of loggerhead turtles recorded associated with a nearshore commercial net fishery at Sandalwood Peninsula, Exmouth Gulf, Western Australia (primary cell values are percentages of all observations).

Table 4. Interaction modes for the different size classes of green turtles recorded in association with a nearshore commercial net fishery at Sandalwood Peninsula, Exmouth Gulf, Western Australia (primary cell values are percentages of all observations).

In contrast with the loggerheads, the green turtle sample data suggest that the smaller sizes classes were more likely to associate with the net fishing work than the larger sub-adult or adult sized turtles. This difference is most evident from comparison of juveniles in the 40 - 49.9 cm CCL size class vs. the turtles of 80 cm CCL and larger. The hand-drag net method trialed was useful for capturing small and medium size juvenile green turtles in particular, but labor intensive, and was not continued (Table 2). The use of targeted netting for turtles in research programs has been efficacious elsewhere (e.g., Boulon 1994; Seminoff et al. 2002).

Our samples were generally acquired as an adjunct to normal commercial fishing operations, and intended as a prelude to a detailed focal study of turtle recruitment, growth and development. That further work has not been supported. Thus, we have no other data to further explore the patterns of association already mentioned, but can confidently state that working cooperatively with fishers provides an avenue to a better understanding of turtles and fisheries interactions and an avenue into further focus on sea turtle habitat use and life history.

Dispersal of turtles into or from within the fishery. This mixed fishery recorded three adult green turtles and one adult loggerhead with known Western Australian nesting locations. All green turtles were recaptures, having first been tagged and released from a nesting beach; two from Barrow Island locations and the other from the Jurabi coast south of North West Cape. The loggerhead turtle was first tagged and released from the fishery, and subsequently discovered nesting on the north end of Dirk Hartog Island. One other female loggerhead turtle from the Dirk Hartog Island nesting group was later discovered (2002) dead in the water off Sandalwood Peninsula by a beachcomber. These data again emphasise the complementarity of nesting beach and in-water investigations for understanding turtle biology. Times elapsed between the nesting and foraging records obtained for these turtles ranged from ~5 - 10 years. Only one of the green turtles had fishery capture records for an extended time: four observations between November 1996 and October 1997 after being tagged at the nesting beach at Barrow Island in November 1986.

Four other juvenile green turtles in the 40 - 49.9 cm CCL group first tagged and released from the fishery were found further afield: one was caught and released from a shark net set by another fisher ~9 months later at a location about 20 km NW of the original capture site. The three other turtles were all found dead. The nearest location recorded for one of these recovered ~12 months after first capture was within Exmouth Gulf ~65 km to the NNW. Post-mortem remeasurement suggested negligible growth since the initial capture. The last two turtles recorded had left Exmouth Gulf. Carcases were recovered on the coast within Ningaloo Marine Park at locations ~160 - 170 km S of NW Cape, and around 250 km from Sandalwood Peninsula; one was found two years and the other was found four years after first capture. Unfortunately, those carcases were not measured. These records emphasise the possible transient occupancy of habitat by some juvenile green turtles transitioning from post-hatchling assumed pelagic foraging mode to a coastal neritic habitat and herbivorous diet, and the possible hazards in making that transition. Similar records of distant dispersal for some juvenile green turtles have been reported from other studies (e.g., Godley et al. 2003; Senko et al. 2010).

Growth and development observations. Our major objective for acquiring this group of tagged predominantly juvenile turtles was to provide a foundation for a more formal study of growth and development, and survivorship of these turtles. Sadly, that focal follow up study of the turtles was not supported. The few growth data we obtained are discussed below.

The loggerhead turtle group included 14 individuals with two or more measurements spanning ≥ 1 year to a maximum of ~6 years. The initial CCL range for these turtles was 68 - 103 cm, although most turtles fell within the initial 70 - 79.9 cm CCL band (Fig. 3). The small number precludes any realistic attempt to examine possible year and seasonality effects, two point data sets are not ideal, and measurement error/variation must be considered. Only one of these sub-adult size turtles (#2891), originally recorded as of indeterminate sex (CCL 79 cm), was found at recapture ~6 years later to have developed the long tail typical of a male, so could then be confidently assigned to the correct sex. CCL had increased to 95.4 cm in that period.

Noting the foregoing, several comments are relevant: (i) generally, multiple measures obtained within periods of ~12 -18 months from first record suggest negligible change in size of any of these turtles; (ii) several of the turtles, still with short tails, and with extended time series observations, (unlike #2891), showed no change in size over periods of >2 years to a maximum ~6 years; (iii) the single turtle of CCL >100 cm at first record (larger than the mean for all adult nesting females from the western Australian regional population) within the group could possibly have been assumed to be an adult, with further substantial growth not to be expected; but the single recapture record almost 4 years later suggests otherwise. The largest female loggerhead turtles within the Western Australian nesting populations are ~115 cm CCL. So, with this individual appearing to grow further, it may be assumed we recorded a likely still immature turtle expected to approach that maximum adult size some time in the future; (iv) short-tailed turtles much smaller than noted above could include individuals maturing at the lower end of the observed size range of adult female loggerheads of the Western Australian nesting populations (see Table 1; but one has since been recorded at 77.5 cm); however, minimal change in size with time might equally indicate prolonged growth stasis of individuals able to survive but not realize their latent growth potential over the relevant periods of observation.

The discussion above highlights the problems inherent in the study of development and recruitment to maturity of turtles in water, even within what might loosely be considered a single habitat. Several loggerhead turtles did however present clear sustained growth records:

Turtle #2891 went unrecorded for ~6 years between captures. CCL increased from 79 cm to 95.4 cm in that period at an average CCL increment = 2.7 cm/yr;

Turtle #2943, with the same ~6 years length of record as #2891, appeared to have reached near maximum size (~96 cm CCL) in ~4 years from first capture at CCL of 75 cm (CCL increment = 4.0 cm/ yr), and thence changed little over the next 2 years.

Turtle #8589 provided a better spread of sample points above 18 months from first capture at CCL of 76 cm cf #2943, but showed a similar general growth trajectory to ~96 cm CCL over ~4 years ( CCL increment = 4.6 cm/yr).

Noting the probable difference in asymptotic adult size for adult male and female loggerheads, the expected range of mature adult size, and the data available, the three cases above show that average growth rates of up to ~4 - 5 cm CCL/year were achieved by some Exmouth Gulf resident loggerhead turtles likely freed from environmental constraint when making the transition from large sub-adult to probable mature size (Bjorndal et al. 2000b; Braun-McNeill et al. 2008).

Similar to the findings of Kubis et al. (2009), our green turtle group was dominated by turtles with single records only (86.8% of total). The turtles with two or more capture records were not always measured, and few of the 57 turtles with two or more measurements available had recapture histories exceeding 12-18 months duration. The limited usable data included a greater spread of initial CCL classes than that for the loggerheads (Fig. 3), but, unfortunately, most cases fall within the group of small to large juveniles only (initial CCL range 40 - 65 cm). Remeasurement data for the larger juvenile to sub-adult size turtles (initial CCL >65 – 80 cm) with long term multiple recapture records were infrequently obtained. None were for turtles in the initial CCL band 70 - 90 cm.

With post-hatchling green turtles entering an initial pelagic life phase whence they appear to grow in carapace length from ~5cmSCLto~35-40cmCCLoverthenext~4-6years,actual carapace growth rates of ~6 - 7 cm/year over this life phase in the wild may be deduced. Bjorndal & Bolten (1988) have reported growth increments of that magnitude or greater for the smallest neritic juvenile green turtles. These small juveniles making the transition from the pelagic into coastal neritic habitat coincidentally make a change from their former predominantly carnivorous diet to the more usual, but not exclusively herbivorous feeding mode (Bjorndal 1997; Nakamura 1980). This habitat transition and diet change certainly imposes challenges for the small juvenile green turtle, and may be anticipated to impact growth rates at least in the short term, before the survivors may proceed to grow and develop toward the next major life change expected at the time of sexual maturity (Miller & Limpus 2003) and beyond. Being ectotherms, and with growth in the field being extended over multiple years and seasons, realized individual growth trajectories of the juvenile and sub-adult turtles are expected to vary widely while also generally conforming to underlying intrinsic patterns. Differences between the sexes might also apply.

Accepting the case above, the limited growth data we acquired for the Exmouth Gulf neritic juvenile green turtles of indeterminate sex suggest a convex pattern of increase in carapace length across the range of initial 40 - 65 cm CCLs, but with a wide possible range of rates for different individuals starting at similar CCLs. The peak of growth rates (Y) within this small sample group appears to be reached between 55 - 60 cm when the individual mean CCL is used as the base x value, and to decline towards 70 cm. The suggested average CCL growth rates for turtles around 42 cm CCL are ~1 cm/ year, for turtles around 59 cm CCL, ~2.5 cm/year, and down to ~1 cm/year again for 72 cm CCL. This is similar to size specific growth patterns for juveniles green turtles reported from more extensive data sets from other populations (e.g., Balazs & Chaloupka 2004; Chaloupka et al. 2004; Kubis et al. 2009; Watson 2006).

Kubis et al. (2009) also showed growth rates for juvenile green turtles from different habitats within a relatively restricted geographic range may differ appreciably, although size specific patterns might show similarities. Their results were derived from analysis of a more substantial data set than ours, and one that also allowed exploration of factors affecting realized patterns of growth. Being unable to carry our Exmouth Gulf population work forward in that way, we have not attempted any more formal analysis of our dataset. We however note that some individuals in the 50-60 cm CCL range achieved shorter term growth rates of ~3-4 cm/yr, while others appeared to be in growth stasis for similar periods. Such individual differences in growth patterns may readily be accommodated by juvenile green turtles where environmental resources and/or population density may vary (e.g., Bjorndal et al. 2000a).

Disease. Fibropapillomatosis (FP) disease has been reported from Western Australia (Raidal & Prince 1996). External soft tissue growths were noted on 25 of the Exmouth Gulf juvenile or sub-adult sized green turtles. The field notes recorded, including the locations and combination of areas affected and the types of lesions seen affecting some of these turtles were consistent with cases of severe FP. Further detailed investigation was not then practicable. Apparently diseased turtles were generally of 45 - 55 cm CCL, consistent with the view that FP is acquired after the juvenile turtles settle into neritic habitat (Ene et al. 2005). This conclusion is further supported by observations on four individuals with extended capture histories where FP apparently developed at periods of 18 months or more after first record. Unusual numbers of sick and dying turtles not included within our tagged group were noted on a number of other occasions, without any obvious causes. Other tagged turtles were known to have died on dispersal away from Sandalwood Peninsula waters, but could not be accessed for necropsy. Debilitated juvenile green turtles afflicted with the “floating syndrome”/buoyancy disorder (Norton 2005) collected from other Western Australian locations were examined by Raidal et al. (1998), who concluded that infections caused by salmonellae, E. coli and other Gram-negative bacteria should be considered as causes of systemic illness and death in wild green sea turtles infected with spirorchid cardiovascular flukes and other internal parasites. There is scope for more focused investigations of health and well- being of sea turtles in Western Australia.

Postscript. Tropical Cyclone Vance (Category 5) devastated the Exmouth Gulf region on 22 March 1999. A c.5 m storm surge associated with the cyclone passage could have driven turtles and other animals inland to strand and die. The magnitude of such possible loss of animals could not be investigated, but that event effectively terminated the possible onward carriage of this field program at that time.

Conclusion. Application of knowledge of sea turtle populations toward better management in the real world requires engagement of the public. Developing that necessary knowledge is not easy, given the common scarcity of funding for research, the labor-intensive nature of in-water sampling in particular, and the need for sustained focus in the field due to the slow growing long lived life history characteristics of these turtles. One of the groups of people that needs to be engaged in seeking these better outcomes for sea turtles are the commercial fishers. Establishing common ground for discourse and learning requires patience and understanding. Our report is a result of following on that path of cooperation. It is unfortunate that support needed for further study of this large group of marked turtles with minimal fuss at low cost could not be secured. Thus, our current goal is to summarize and present the information that can be extracted from records obtained.

We have shown that commercial fishing is not necessarily detrimental to sea turtle conservation, and may contribute positively to addressing data needs. With the passage of time since our marked turtles were last accessed, and given the likelihood of tag losses from the growing juvenile turtles (Prince 1996) over the past decade, it might not be possible to easily reconnect to that population if resources could be secured. However, we still lack necessary demographic data for juvenile turtles of all species known breeding in the western Australian region. These populations include the major portion of the SE Indian Ocean sea turtle stocks (Baldwin et al. 2003; Dethmers et al. 2006). Correcting that data deficiency should be a priority. Engagement of commercial fishers and other industry and public interest groups, including indigenous hunting communities in that work should be integral to any such enterprise.

Acknowledgments. We thank the anonymous referees who commented on the draft that was circulated for their suggestions; and colleagues who checked some references that were proving difficult to recall. Anthony Hart’s contribution of unpublished data in Figure 2 is appreciated. We thank the MTN editors and Michael Coyne for help in getting this into final copy.

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