The potential effects of climate change on southern calamary in Tasmanian waters: biology, ecology and fisheries



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The potential effects of climate change on southern calamary in Tasmanian waters: biology, ecology and fisheries
Short paper produced for the World Wildlife Fund U.S.
Gretta T Pecl1 and George D Jackson2


1Tasmanian Aquaculture and Fisheries Institute

University of Tasmania

Private Bag 49

Hobart, Australia 7001


2Institute of Antarctic and Southern Ocean Studies

University of Tasmania

Private Bag 77

Hobart, Australia 7001


1 Corresponding author

Email: Gretta.Pecl@utas.edu.au

Phone: +61 3 62277277

Fax: +61 3 62278035



Summary


Virtually every facet of squid life-history that has been examined thus far has revealed an incredible capacity in this group for life-history plasticity. The extremely fast growth rates of individuals and rapid rates of turnover at the population level mean that squid can respond quickly to environmental or ecosystem change. Their ‘life-in-the-fast-lane’ life-style allows them to rapidly exploit ‘vacuums’ created in the ecosystem when predators or competitors are removed. In this way they function as ‘weeds of the sea’ (Jackson and O’Dor 2001). Elevated temperatures accelerate the life-histories of squid, increasing their growth rates and shortening their life-spans. At first glance, it would be logical to suggest that rising water temperatures associated with climate change (if food supply remains adequate) would be beneficial to inshore squid fisheries – growth rates may increase, squid would be larger, and turnover of populations more rapid. However, the response of inshore squid populations to climate change is likely to be extremely complex. The size of hatchlings emerging from the eggs becomes smaller as temperatures increase, and hatchling size may have a critical influence on the size-at-age that may be achieved as adults, and subsequently population structure. The influence of higher temperatures on the egg and adult stages may thus be opposing forces on the life-history. The process of climate change will likely result in squids that hatch out smaller and earlier, undergo faster growth over shorter life-spans, and mature younger and at a smaller size. Individual squid will require more food per unit body size, require more oxygen for faster metabolisms, and have a reduced capacity to cope without food.




Introduction

Cephalopod populations in our oceans appear to be expanding as world fisheries remove their fish competitors and predators (Caddy and Rodhouse 1998). An appreciation of the role of this group in the world’s marine ecosystems can be achieved by considering just how many squid, octopus and cuttlefish there are in the ocean. The total biomass of this group has been estimated to equal that of all species of fishes in the world’s oceans (Clarke 1987)! Many different cephalopod species are targeted commercially, however, it is the migrating oceanic ommastrephid squids and the inshore coastal loliginid squids that form the basis of major cephalopod fisheries, and also play very important roles in the trophic structure of the worlds marine ecosystems (Rodhouse and Nigmatullin 1996).


In many ways, squids are ecological equivalents to teleost fish. However, their physiology, biochemistry, and life-histories are very different to their teleost competitors (O’Dor and Webber 1986). Squids are geared to doing everything fast, with extremely fast growth rates, short life-spans and therefore rapid turnover of populations. Squid life-spans are a fraction of their teleost competitors - while some fish are now known to live for over a century, squids generally rarely live longer than a year, and sometimes considerably less (Jackson in press).

Squids have some of the highest growth rates of poikilothermic animals, several times greater than that of fish and approaching homeothermic mammals (Lee 1994). To fuel such incredible growth throughout their short lives (typically <1 year), their metabolic rates are rapid (Seibel et al. 1997), and their appetites ravenous. Adult temperate species consume up to 30% of their body weight per day in food, and juveniles up to a massive 72% (Segawa 1990). Unlike most fish, squids and other cephalopods have continuous growth throughout their life-cycle without reaching an asymptotic size (Alford and Jackson 1993). At young ages at least, growth is exponential where the growth trajectory concaves upwards and very small differences between individuals in the early growth stages amplify throughout the life-span (working like compound interest). As a group, cephalopods are becoming renowned for the extreme flexibility and plasticity of their life-histories, largely a result of dramatic and direct responses of growth to temperature. This flexibility is largely a function of the highly responsive nature of squid physiology to environmental elements (Jackson and O’Dor 2001), with temperature often cited as a key driving factor (Forsythe 1993). For example, Loligo forbesi hatchlings reared at two temperatures with only 1oC difference resulted in squid that were three times larger in the warmer group after 90 days than the cooler reared siblings (Forsythe and Hanlon 1989).


With such profound and direct effects of temperature on the life-cycles and life-history of squids, our emerging understanding of the crucial role of cephalopods in many ecosystems, and the increasing commercial importance of this group, it is little wonder that many researchers have begun to ponder the potential effects of climate change on squid populations and fisheries. Forsythe et al. (2001) asked the obvious question of how squid populations will react to ocean scale temperature change and urged future research in this direction. Given that many species seem to tolerate and even thrive in warm conditions it has been suggested that cephalopods, inshore loliginid squid in particular, will prosper with global warming. Increased growth rates, accelerated life-histories, and rapid turnover in populations could potentially lead to population expansion at the expense of slower growing teleost competitors (Jackson in press). As short-lived species with plastic growth and reproduction, and high mobility, they would better poised than many species or groups to respond to changed balances in their environment (Boyle and Boletzky 1996). As a group, their potential to adapt in one way or another will undoubtedly prove true, however, is our current perception of the way squid populations may respond to climate change too simplistic?
By the end of the next century global mean sea surface temperatures (SST’s) are expected to rise substantially (1-3.5oC, Watson et al. 1996 in Forsythe in press; 1.4 to 5.8 oC Schneider 2001). However, climate change may mean more than just temperature rises. Other predictions with potential impacts on squid populations include an increase in extreme events with more intense El Niño events possible, and more common El Niño like conditions (Easterling et al. 2000). Rises in atmospheric CO2 are also expected, increasing surface ocean CO2 concentrations and resulting in an estimated drop in pH of about 0.4 units, which may inhibit oxygen uptake of squids (Seibel and Fabry 2003). Abiotic changes in the world’s oceans will result in concomitant changes in the biotic components as well. Global warming is expected to increase thermal stratification of the upper ocean thereby reducing the upwelling of nutrients and decreasing productivity (Seibel and Fabry 2003). Indeed, the warming of some oceans has already been accompanied by a 70% decline in zooplankton abundance (Roemmich and McGowan 1995). There appears to be a non-linearity in the response between physical and biological processes, in that a small change in temperature can be amplified into profound change in the abundance of zooplankton, micronekton and its predators (Veit et al. 1997). Changes in temperature will also have indirect effects in addition to direct impacts on the metabolism of species, affecting the abundance and activity rates of predators (Bailey and Houde 1989).
The cumulative effects of climate change on marine ecosystems are already evident with populations showing changes in the timing of life-cycle events (Beaugrand and Reid 2003) and shifts towards higher latitudes according to thermal preferences (Daufresne et al. 2003). There has also been widespread documented biological changes in the 20th century including changes in species abundance, distribution, morphology, behaviour and community structure (Easterling et al. 2000). Changes in the distribution of cephalopods has already been noted, with the sudden appearance of subtropical and tropical species in temperate Galician waters – including the squid Alloteuthis Africana and the common paper nautilus Argonauta argo, an effect attributed to increase in SST of north-eastern Atlantic (Guerra et al. 2002).
The purpose of this discussion paper is to explore how climate change may impact on the life-history characteristics and population dynamics of inshore squid, and ultimately impact on our inshore squid fisheries. Recently there has been much research into the life-history and fisheries biology of the southern calamary, Sepioteuthis australis. This is now one of the better understood inshore squid species in the world, and we therefore use this species as a case study.




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