Will Fast Growing Squid Replace Slow Growing Fish?
Dr G D Jackson
Senior Lecturer, Institute of Antarctic and Southern Ocean Studies, University of Tasmania
What is the significance of squid in the world's oceans and especially in the Southern Ocean? Squid are often overlooked when analysis is made of the marine environment, however, their importance in the ecosystem is becoming increasingly recognised. Squid belong to the group of invertebrate molluscs referred to as cephalopods. The most important and abundant cephalopods belong to the three major families of octopus, squid and cuttlefish. Australia and the associated Southern Ocean have a great diversity of species and considerable stocks of many cephalopods. Squid are the dominant cephalopod in the open pelagic environment over the continental shelves, slopes and in the open ocean, including the deep sea. Squid are voracious eaters, all are predators and cannibalism is not uncommon. In the marine food web they serve as both important predators and prey. While they prey on many fish, crustacean and cephalopod species they in turn serve as important prey for a variety of vertebrate predators (fish, mammals and birds).
We can get an idea of the abundance of squid in the world's ocean by considering the consumption of cephalopods (mainly squid) from just one cephalopod predator the sperm whale. Sperm whales alone are estimated to consume in excess of 100 million tonnes of cephalopods a year. This is equivalent to the total world fishery catch and probably exceeds half the total biomass of mankind on the earth (Clarke 1983). It is therefore highly likely that the standing biomass of squids within the world's oceans probably exceeds the total weight of humankind on the earth. Given such importance squid have generally not been given the attention they deserve or have not been incorporated to the degree they need to in ecosystem models. Future research needs to rectify this.
Squid life histories - life in the fast lane
Squid are not only important in the marine ecosystem, they are unique both biologically and physiologically. A major breakthrough that has aided both our understanding of squid population dynamics and fisheries management has been the discovery of a means to age individuals using daily statolith increments. Statoliths are essentially small balance 'bones' that sit at the back of the squid head. An important discovery has been that statoliths grow in layers much like an onion or tree rings. A variety of experiments undertaken my both myself and others have been able to prove experimentally that the rings or increments are in fact laid down daily (eg Jackson 1994, Jackson et al 1997). Squid thus have a 'calendar' in their head that records when they hatched, how old they are and what their growth rate has been through their life cycle. This tool has revolutionised squid biology.
Squid statolith analysis has resulted in two profound findings: Firstly squid don't live very long and secondly squid have a unique form of growth that is continuous and non-asymptotic. Based on a review of the literature and continuing research (Jackson 1994, Jackson 1998, Jackson & O'Dor submitted) squid have short life spans that are measured in days not years. For example I have not found any tropical Australian squid that is older than 200 days and some small tropical species complete their life cycle in just a couple of months. Cooler species appear to be annual. There is not much evidence for squids living longer than a year, possibly only a few species. Most life cycles seem to be between about 2-12 months. Don't ask me about giant squid, this discussion always leads to the question 'what about giant squid?' I don't know but from what data is around they may be only a couple of years old.
Figure 1: Diagrammatic representation of fish growth versus squid growth. Fish can only recruit muscle fibres for a limited period (hyperplasia) and then increase the size of those muscle fibres (hypertrophy). However, squid can grow by both hyperplasia and hypertrophy throughout their life span.
The form of growth of squid is also unique and interesting. Squid just keep growing. They do not show the distinctive flattening in their growth curve shown by their fish competitors. Many species growth can be modeled with exponential or linear curves. The interesting thing is they continue growing even during their maturation phase until they die or are eaten. They seem to achieve this because of a number of unique qualities, (1) they have a protein based metabolism with efficient digestion so food is converted to growth rather than stored, (2) they are efficient feeders, using their suckered arms and beak they can remove only the highly digestible parts of prey and 'spit out the bones' and (3) they can grow by continually increasing the number of their muscle fibres (hyperplasia) a feature not shared by their fish counterparts. While juvenile fish recruit new muscle fibres by hyperplasia they reach a point where growth only occurs by increasing the size of existing muscle fibres (hypertrophy). This probably contributes to their flattening growth curve. Alternatively, squid show both hyperplasia and hypertrophy throughout their life span, thus they continue to recruit new fibres as well as increase the size of existing fibres (Figure 1). Such a strategy might account for their continuous growth. All of the above features contribute to the unique form of growth and the ability of squid to grow fast and fill available niches. Their life is very much life-in-the-fast-lane. They are the 'weeds' of the sea.
World Fisheries
What about world fisheries, where do squid fit into the picture? If we consider the world fishery situation the prognosis is not good, in fact depressing. The global trend is that we are 'fishing down marine food webs' (Pauly et al 1998). The major thesis of the work by Pauly and co-workers, published in Science is that as fisheries remove the larger fish-eating predators, effort is being shifted to invertebrates and smaller plankton-eating fish. In each passing decade the trophic level of fisheries continues to decline with exploitation at lower and lower trophic levels. Pauly et al (1998) questioned such a strategy and suggests the present trends will lead to widespread fisheries collapses. Will world fisheries be eventually harvesting plankton?
Figure 2: Trends (% change from 1970 values) in the world catch of traditional groundfish and cephalopods from 1970-1994 (from Caddy & Rodhouse 1998).
Where do squid fit into the picture? Interestingly, squid and cephalopods generally show a very different trend. Caddy & Rodhouse (1998) provided a synopsis of cephalopod fisheries in relation to world catches. The comparison of cephalopod catches with traditional groundfish landings reveals a stark contrast. Cephalopods are one of the few remaining marine groups where there is still an increase in landings compared to the decrease in many finfinsh. If we look at the percent increase in cephalopod catches compared to groundfish landings the curves over the last three decades are quite divergent (Figure 2). While finfish catches have remained fairly stable or declined, the catch of cephalopods has increased substantially. This trend has been suggested to be due to both a removal of cephalopod predators such as toothed whales and tunas and an increase of cephalopods due to the removal of finfish competitors.
In consideration of tuna consumption alone, Caddy & Rodhouse (1998) pointed out how tuna landings have risen from two to four million t/year. Given that tuna diet is approximately 25% oceanic squids and consumption is about 10% body weight/day, this two ton difference accounts for an extra 20 million/t of squid in the world's oceans in recent years. Just as fast growing weeds can quickly colonise an area of ground that has been denuded of vegetation, the rapid growth of squids and short life cycles have enabled them to move into regions that have been heavily overfished.
Looking South, squid in the Southern Ocean
In the vast Southern Ocean, squid possibly play a more important role there than in other regions of the ocean. Squid are an integral component in both the Antarctic and Sub-Antarctic oceans. Next to krill, squid are the most important invertebrate group in Antarctic waters. Furthermore, in recent years, a whole unknown trophic system in the Antarctic Polar Frontal Zone (APFZ) was described in which squid replace fish and where krill are not part of that particular food web. In the Atlantic Sector of the APFZ Rodhouse & White (1995) have pointed out how the ommastrephid squid Martialia hyadesi is the predominant predator in the Scotia Sea and the food chain consists of phytoplankton ¨ copepods ¨ mesopelagic myctophids (lanternfish) ¨ ommastrephid squid ¨ vertebrate predators. It is likely that such a food chain exists in other regions of the Southern Ocean but has not yet been identified. Diet studies of the onychoteuthid squid Moroteuthis ingens around New Zealand (Jackson et al 1998) and around Macquarie and Heard Islands (Phillips et al 2001) all have indicated this same food chain except the squid is a demersal onychoteuthid rather than a pelagic ommastrephid. Phillips et al (2001) suggested that this copepod-myctophid-squid-higher predator food chain may be one of the most widespread and overlooked food chains in the Southern Ocean.
Such recent findings within this last decade highlight how much we have to learn in the Southern Ocean. This is especially true in the southern Indo-Pacific which includes the Australian sector of the Southern Ocean. In recent years our information has increased in temperate and sub-Antarctic waters more quickly than in Antarctic waters.
Moroteuthis ingens a squid success story
Much of my Southern Ocean research has focused on the warty squid Moroteuthis ingens. Up until recent years this species was poorly understood and delegated to obscurity due to lack of biological information. However, this species is regularly caught in both fishing and research trawls and my research has focused on New Zealand, The Falkland Islands and more recently Australia's sub-Antarctic island regions. The biological understanding of this species is now perhaps the best of any sub-Antarctic squid. It is a large squid growing to over 500mm in mantle length and females achieve a much larger size than males. While M. ingens is epipelagic during its juvenile stage it undergoes an ontogentic descent to take up a demersal existence (Jackson 1993). This species has a biologically unusual and interesting reproductive strategy referred to as terminal spawning (Jackson & Mladenov 1994). Although it is a muscular squid, females (and to a lesser extent males) undergo a dramatic change associated with reproduction. Females produce a huge ovary that can reach the size of a rugby ball and weigh as much as a kilogram. In fact the ovary can weigh more than the total body weight of the male. In association with the development of the ovary the female undergoes a dramatic tissue breakdown in its body wall. This process results in a total loss of muscle fibres that transforms the muscular female into something more analogous to a jellyfish and death is associated with spawning. Moroteuthis ingens and other onychoteuthids are important prey for a number of vertebrate predators (at least four mammals, 17 birds, 13 fish, Jackson et al 1998). It is suspected that this tissue breakdown may result in dead individuals floating to the surface where they are accessible to mammals and birds.
The statolith ageing of this species suggests that its life cycle is only around a year (Jackson 1997). Furthermore, there appears to be two prominent zones in its statoliths that correspond to the different environments this squid inhabits during its life cycle. As an adult, it is a deepwater species that is predominantly found in waters deeper than 300m off southern New Zealand (Jackson et al 2000) but is found in shallower waters on the Patagonian Shelf (Jackson et al 1998) presumably due to the cooler water temperature there. Recent comprehensive distributional data in New Zealand (Jackson et al 2000) has revealed strong niche separation between warty squid and New Zealand arrow squid Nototodarus sloanii. Warty squid are restricted to deeper, cooler sub-Antarctic water while the arrow squid are shallower inhabiting sub-tropical water. The subtropical front is a major oceanographic feature separating these two species and appears to act as a barrier (Figure 3). The biomass of the New Zealand arrow squid per unit area is generally much greater than warty squid, however, the distribution of M. ingens in deep water of the sub-Antarctic region appears to be ubiquitous. There appears to be a thin M. ingens 'carpet' spread across much of the ocean floor circum-globally in the sub-Antarctic region.
Figure 3: A three dimensional graph of the abundance of both (a) Nototodarus sloanii (arrow squid) and (b) Moroteuthis ingens (warty squid). The graphs depict the mean catch of squid in separate research trawls undertaken in southern New Zealand waters. For perspective the viewer is standing in the Southern Ocean and looking up to the South Island of New Zealand. The distribution of arrow squid show a sharp demarcation against the 500m contour which is the location of the subtropical front. Warty squid in graph b are shown to be abundant in water predominantly deeper than 500m in sub-Antarctic waters (from Jackson et al 2000).
As fisheries look south
Large-scale fishing operations continue to look south for new stocks, especially as more traditional stocks become depleted. What does this mean for squid stocks within the Southern Ocean? Firstly, we need to find out what species are abundant and what is their potential biomass. Much of the pelagic environment is largely unexplored. It has been suggested that in the Antarctic waters alone, standing stock of squid may be as high as 100 million tonnes with vertebrate predators consuming around 30 million tonnes annually (Clarke 1983). It is likely that the Antarctic Polar Frontal Zone in Australia's sector of the Southern Ocean harbours large stocks of ommastrephid squid as it does in the South Atlantic (Rodhouse & White 1995). We just don't know the details of squid stocks in our region of the Southern Ocean. There is potentially an enormous biomass there that we don't yet know about. Both legal and illegal fishing for Patagonian toothfish could dramatically change the environment in the Southern Ocean as fishing for other species has populations could have the potential for replacing toothfish populations and other populations of sub-Antarctic target finfish. Furthermore, squid also respond dramatically to even slight increases in water temperature with a rapid increase in growth in association with a temperature increase. An increase in water temperature due to global warming could also favour population expansion of squids over fish.
What we need now is a better understanding of the biology, abundance and biomass of Southern Ocean squids. Such data is urgently needed for ecosystem modeling and ecosystem management. Such data gathering requires input from 'big science'. There needs to be focused squid sampling in key regions of the Southern Ocean by both large trawling and targeted jigging operations in association with GIS data gathering to tie in the oceanography with pelagic biology. Concentrating scientific fishing efforts in the Antarctic Polar Frontal Zone would probably be a good place to start.
To achieve these aims, we need to find funding for large scientific programs to probe into the pelagic environment of the Southern Ocean. Such studies are costly but necessary. These large-scale research projects might carried out through collaborative funding from both Australian agencies (eg The Australian Research Council) and overseas funding agencies (eg Census of Marine Life, Washington DC).
References
- Caddy, J.F. & P.G. Rodhouse (1998). Cephalopod and groundfish landings: evidence for ecological change in global fisheries? Rev. Fish. Biol. & Fisheries 8: 431-444.
- Clarke, M.R. (1983). Cephalopod biomass ø estimation from predation. Mem. Natl. Mus. Vict 44: 95-107.
- Jackson, G. D. (1993) Growth zones within the statolith microstructure of the deepwater squid Moroteuthis ingens (Cephalopoda: Onychoteuthidae): evidence for a habitat shift? Can. J. Fish. Aquat. Sci. 50: 2366-2374.
- Jackson, G.D. (1994) Application and future potential of statolith increment analysis in squids and sepioids. Can. J. Fish. Aquat. Sci. 51: 2612-2625.
- Jackson, G.D. (1997) Age, growth and maturation of the deepwater squid Moroteuthis ingens (Cephalopoda: Onychoteuthidae) in New Zealand waters. Polar Biol. 17: 268-274.
- Jackson, G.D. (1998) Research into the life history of Loligo opalescens: where to from here? Calif. Coop. Fish. Invst. Repts. 39:101-107.
- Jackson, G.D., J.W. Forsythe, R.F. Hixon & R.T. Hanlon (1997) Age, growth and maturation of Lolliguncula brevis (Cephalopoda: loliginidae) in the Northwestern Gulf of Mexico with a comparison of length-frequency vs. statolith age analysis. Can. J. Fish. Aquat. Sci. 54: 2920-2929.
- Jackson, G.D., M.J.A. George & N. G. Buxton. (1998) Distribution and abundance of the squid Moroteuthis ingens (Cephalopoda: Onychoteuthidae) on the Patagonian Shelf region of the South Atlantic. Polar Biol. 20: 161-169.
- Jackson, G.D. & P.V. Mladenov (1994) Terminal spawning in the deepwater squid Moroteuthis ingens (Cephalopoda: Onychoteuthidae). J. Zool. Lond. 234: 189-201.
- Jackson G.D. & R.K. O'Dor (submitted) Time, space and the ecophysiology of squid growth, life in the fast lane. Vie et Milieu.
- Jackson, G.D., A.G.P. Shaw & C. Lalas. (2000). Distribution and biomass of two squid species off southern New Zealand; Nototodarus sloanii and Moroteuthis ingens. Polar Biol. 23: 699-705.
- Pauly, D., V. Christenses, J. Dalsgaard, R. Froese, F. Torres Jr. (1998). Fishing down marine food webs. Science 279: 860-863.
- Phillips, K.L., G.D. Jackson & P.D. Nichols (2001) The diet of the sub-Antarctic squid Moroteuthis ingens around Macquarie and Heard Islands: stomach contents analyses and fatty acids as dietary tracers. Mar. Ecol. Prog. Ser. 215:179-189.
- Rodhouse P.G., & M.G. White (1995) Cephalopods occupy the ecological niche of epipelagic fish in the Antarctic Polar Frontal Zone. Biol. Bull. 189: 77-80.
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