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The biology and prevalence of Cirolana hesperia and their effect on the Western Rocklobster Fishery

Winzer, Andrew Charles (2007) The biology and prevalence of Cirolana hesperia and their effect on the Western Rocklobster Fishery. PhD thesis, Murdoch University.

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The current study had two overriding and inter-related aims. The first was to describe the biology of C. hesperia the dominant scavenging subtidal cirolanid found in temperate waters off the Perth metropolitan coastline and thereby extend our knowledge and test several hypotheses regarding the biology of scavenging crustaceans in general. The second was to use the biological information generated to develop fishing practices that reduce the amount of bait used by trap based fisheries without reducing the overall catch. Fulfilling the latter aim has the potential to greatly reduce some of the deleterious effects of fishing practices around the world.

A survey was designed to determine whether the anecdotal evidence that commercial rocklobster fishers believe sea lice have a negative impact on the ratio of bait used to rocklobster landed was widespread across the fishery and, if so, whether they associated particular factors with increases or decreases in lice numbers. The response rate of over 40% was high for a survey of this type and had similar return rates across all zones of the fishery. Such a high return rate, in addition to the responses confirms that commercial fishers, regardless of zone, consider that sea lice have adverse effects on the ratio of bait used to rocklobster landed.

The findings of the survey allowed the following conclusions to be drawn and hypotheses developed. Commercial fishers believe particular temporal and spatial factors are associated with increased numbers of sea lice predating on rocklobster baits. For example, fishing in depth ranges of 20 - 30 fathoms (40 – 60 metres) and greater, fishing on a sand bottom, fishing during the months of November, December, March and April, fishing during the full and new moons and lastly during periods of weak current and high water temperature were all felt to increase the number of sea lice scavenging on baits.

Laboratory growth trials demonstrated significant differences in the mean body lengths of starved and fed manca and adult C. hesperia after three and seven months, respectively. Despite these findings, starved adult C. hesperia continued to moult and grow after approximately 300 days of fasting. The relatively short period of time before differences in starved and fed manca C. hesperia growth parameters arose is probably due to early life stages of cirolanids being more reliant on regular food sources than their adult counterparts. An increase in water temperature from 16 ºC to 24 ºC resulted in no significant increase in mean body length of manca C. hesperia. However, the mean inter-moult period of fed manca housed at 24 ºC was significantly lower (25-50%) than both the fed treatment group housed at 16 ºC and the starved treatment group within the first two months of the trial. This reduction in mean inter-moult period is indicative of an animal which is growing quickly as a result of increased metabolic rate albeit at reduced increments.

The 24 hour activity rhythms of C. hesperia in shallow waters in the metropolitan zone of the Western Rocklobster Fishery were determined in order to identify those factors which modulate the emergence and/or swimming activity of this dominant scavenger. It was concluded that the presence of a circadian rhythm of spontaneous nocturnal activity induced by the end of the diurnal period of photo-inhibition,regardless of tidal regime or lunar cycle, is evident in adult C. hesperia. In contrast, circa-tidal rhythms of emergence centered around spring tides were found to exist in juvenile C. hesperia with similar mean numbers trapped across all sampling times (see Chapter 3, tables 3.4-3.8) with the exception of high water spring tides (HWS), which resulted in significantly greater mean numbers. It was concluded that mature cirolanids found off the Western Australian coastline adopt a strictly nocturnal and facultative necrophage mode of existence despite carrion generally being regarded as a much less crucial food resource in relatively shallow, productive, inshore waters. However, juvenile cirolanids found off the Western Australian coastline were recognised as relying on a more opportunistic scavenger mode of existence being active during both day and night. This perpetual foraging strategy employed in cirolanids at manca and juvenile stages is possibly due to the lack of the well developed chemosensory and or olfactory capabilities found in their adult stages. In addition to this reduced foraging efficiency, a smaller gut size combined with higher metabolic rates and lower assimilation efficiencies ensures their ongoing search for sustenance.

Analyses of the number of C. hesperia collected during three years of trapping in C zone of the Western Rocklobster Fishery clearly demonstrated that the numbers of cirolanids caught in rocklobster pots were significantly affected by bottom type, moon phase and depth. With respect to bottom type, the greatest numbers of C. hesperia were trapped in rocklobster pots deployed on weed/rock and sand bottoms while significantly lower numbers of C. hesperia were trapped on rock or weed bottoms. In regards to moon phase, rocklobster pots deployed during the new and last quarter of the moon phase trapped significantly greater numbers of both C. hesperia and the Western Rocklobster, P. cygnus than those deployed during the full and first quarter phases. Rocklobster pots containing lice traps deployed in depths ranging from 10-19 and 20-29 fathoms (20-39 and 40-59 metres respectively) caught significantly greater numbers of C. hesperia and P. cygnus than pots deployed in the 1-19 metre depth range. Lastly, the catchability of both C. hesperia and P. cygnus was greatest when either Australian salmon or blue mackerel was deployed in pots indicating these scavengers do exercise a degree of selectivity with respect to particular bait types.

Whilst ovigerous C. hesperia were present in each month of the year, peaks in their abundance and/or catchability occurred at the height of summer (January to March). These peaks in mpm, i.e. morphological parturial moult stage, C. hesperia were then followed by corresponding peaks in recruitment from April to June. I hypothesise that peaks in the reproductive intensity of C. hesperia can be attributed to peaks in water temperature and also the seasonal influx of carrion due to both recreational and commercial charter fishing and the commencement of the rocklobster season all occurring predominately within a similar time frame, i.e. the warmer months.

The current study has clearly demonstrated that a plethora of factors attract high numbers of both C. hesperia and P. cygnus. Thus, it is apparent that those practices employed by commercial fishers to land the greatest number of P. cygnus will also attract significant numbers of C. hesperia to rocklobster baits. Irrespective of key factors driving C. hesperia abundance, the high market price received for P. cygnus in conjunction with the relatively low price payed for bait, ensures fishers will continue to deploy rocklobster pots in the manner which maximises their landings. Thus for a reduction in bait usage to offer significant economic savings to the industry, different methods of bait deployment that do not result in a decrease in the catch of P. cygnus must be developed.

A series of baitsaver trials were conducted in order to determine whether baitsavers deployed containing reduced quantities of bait would catch the same numbers of rocklobster as conventional pots using the typical amounts of bait employed in the fishery. During the start of the season the majority of fishers concentrate their efforts in shallow waters and employ predominantly one day soaks as they target rocklobster migrating from the shallows to offshore habitats (whites phase – November to February). In the later parts of the season (reds phase – March to June) fishing effort becomes less concentrated as some fishers continue to fish the inshore grounds whilst others move out into deeper waters. The season long trial demonstrated that packing large quantities of bait in both baitboxes in the pot (traditional method) caught significantly more rocklobster than pots using a baitsaver in one bait box while the other bait box was packed in traditional fashion (combined method) during the first two months of the whites phase. However, during the remainder of the season there was no significant difference in catches between the two methods. Thus, the season-long trial found that the use of traditional methods of bait deployment in the first two months of the season in conjunction with the combined method of bait deployment from January until the end of the commercial season has the potential to save inshore and offshore based fishers $34,000 and $36,000 each year.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Gill, Howard
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