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Keywords:

  • fisheries management;
  • growth;
  • Irish Sea cod;
  • maximum sustainable yield;
  • recruitment

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

Environmental factors act on individual fishes directly and indirectly. The direct effects on rates and behaviour can be studied experimentally and in the field, particularly with the advent of ever smarter tags for tracking fishes and their environment. Indirect effects due to changes in food, predators, parasites and diseases are much more difficult to estimate and predict. Climate can affect all life-history stages through direct and indirect processes and although the consequences in terms of growth, survival and reproductive output can be monitored, it is often difficult to determine the causes. Investigation of cod Gadus morhua populations across the whole North Atlantic Ocean has shown large-scale patterns of change in productivity due to lower individual growth and condition, caused by large-scale climate forcing. If a population is being heavily exploited then a drop in productivity can push it into decline unless the level of fishing is reduced: the idea of a stable carrying capacity is a dangerous myth. Overexploitation can be avoided by keeping fishing mortality low and by monitoring and responding rapidly to changes in productivity. There are signs that this lesson has been learned and that G. morhua will continue to be a mainstay of the human diet.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

The International Council for the Exploration of the Sea (ICES)-Global Ocean Ecosystem Dynamics (GLOBEC) Cod and Climate Change (CCC) programme ran from the early 1990s until the end of 2009. There were 16 workshops, several symposia and many publications from its component national and regional programmes (Brander, 2010). This paper presents some of the findings from these and reviews what role climate change played in the decline of cod Gadus morhua L. stocks. Experiments and comparative studies can be applied to predict the effects of climate change and improve the management of fish stocks for sustainable production. Gadus morhua stocks occur across the North Atlantic Ocean (Fig. 1) in areas with mean annual bottom temperatures from close to 0° C (Labrador, Gulf of St Lawrence, Greenland) to 11° C (Celtic Sea). The author began his research career working on Irish Sea G. morhua and as the symposium from which this is volume is drawn was held in Belfast, this stock will be used as an example.

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Figure 1. Distribution of Gadus morhua across the North Atlantic Ocean. inline image, spawning areas; inline image, transport in pelagic early life; inline image, return migrations; inline image, overwintering areas for Calanus finmarchicus, the principal prey species for G. morhua larvae (joint copyright G. Gorick and GLOBEC).

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Experiments on the physiology and behaviour of individual fishes are vital for understanding the processes that govern population dynamics (Pörtner, 2010), but may be difficult to apply to populations in the sea (Guderley et al., 1996). Unlike agriculture and other terrestrial systems, the scope for in situ, large-scale replicated marine experiments is very limited. Fishing and other anthropogenic activities can be treated as unintended experimental perturbations and their consequences can be analysed. It is rarely possible, however, to compare such unintended experiments with clear-cut controls in order to eliminate possible confounding factors. The advent of marine protected areas must be seized on as an opportunity to learn from experimental perturbation (or the removal of previous perturbation).

The comparative method (making interstock comparisons) has been extensively used in the CCC programme and was a major influence on its design and justification; regional studies on G. morhua were already in progress throughout the range of the species and it was clear that much could be learned by their comparison (ICES, 1993). A much better picture is now available of the response of individual growth and population recruitment to change in temperature from studies that bring together information over the whole thermal range of the species (Andersen et al., 2002; Dutil & Brander, 2003; Rätz & Lloret, 2003; Mieszkowska et al., 2009). A comparison of growth rates observed in the sea in different areas with those measured in experimental systems shows up the strengths and shortcomings of both approaches (Buckley et al., 2004; Folkvord, 2005).

During the course of the CCC programme, concerns over climate change greatly increased and this had a considerable effect on science strategy and priorities. Thus, at the start of the programme, the emphasis was mainly on how interannual and decadal variability in environment affected G. morhua recruitment, but the emphasis has now shifted to study long-term consequences of the anthropogenic fraction of climate change.

Irish sea g. morhua: An example of a declining stock

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

The major elements of the population dynamics of G. morhua in the Irish Sea (growth, recruitment, mortality, stock structure, movements and trophic relationships) have been known since 1975 (Brander, 1975), although a great deal of detail and background knowledge about the Irish Sea marine ecosystems has been added since (Brander & Dickson, 1984; Thompson & Harrop, 1991; Fox et al., 2000). A regional structure for science and governance that would bring in all interests and stakeholders was put forward (Brander, 1978). A case for an ecosystem approach to fisheries of the area was made in 1980, which would also integrate activities on other ecosystem services and impacts on the marine environment (gravel extraction, dumping at sea and shipping) (Brander, 1980). The trophic relationships between G. morhua, Nephrops norvegicus and other species in the Irish Sea were used to establish an E.U. multispecies management regime in the 1980s (Brander & Bennett, 1986) and proposals were put forward by Dutch and British negotiators to implement an overall effort limit on fisheries in the area, based on estimates of the surplus production of all demersal species (Brander, 1977). All these are gradually coming into being, but it has taken a long time.

Meanwhile Irish Sea G. morhua has not fared well since 1980. Between 1968 and 1980 the instantaneous annual rate of fishing mortality averaged <0·8, but since then it has been increasing by 0·25 per decade (Fig. 2). This means that the length of time that a fishable-sized G. morhua can expect to avoid being caught has dropped from 1 year to just over 6 months. The recommended precautionary level of fishing mortality (ICES, 2010a) is 0·72. Average total international landings from 1968 to 1980 were >9000 t, peaked at close to 15 000 t in 1981 and have been declining by 500 t year−1 ever since. Because the maximum yield per recruit for this stock occurs at a fishing mortality of 0·31 [Brander (1975) and ICES (2010a) give the same value], it is evident that the stock has been growth-overfished for over 40 years. The stock is also recruitment-overfished; spawning stock biomass is now <10% of the level in 1982 and recruitment is consistently very low (Fig. 2).

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Figure 2. (a) Fishing mortality (F; inline image) and total international landings (inline image) and (b) spawning stock biomass (SSB; inline image) and recruitment (R; inline image) of Irish Sea Gadus morhua since 1968. Data from ICES (2010a).

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The increase in fishing is a sufficient explanation of the decline in Irish Sea G. morhua and this is not the place to discuss the aims and shortcomings of fisheries management in the Irish Sea. Nevertheless, there are questions that need to be addressed concerning the possible role of climate change in the decline of G. morhua in this and other areas. Would the decline have taken place in any case, even if fishing mortality had been restrained? Could a decline in surplus production coupled with increased catchability and fishing power (i.e. greater mortality per unit of fishing effort) have contributed to the increase in fishing mortality?

Direct effects on individuals and populations

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

Gadus morhua in the Irish and Celtic Seas are at the upper thermal boundary of the stock and their growth and surplus production (i.e. including the reproductive component) are among the highest found throughout the range of this species (Fig. 3) (Dutil & Brander, 2003; Rätz & Lloret, 2003). Icelandic experiments on individual fish fed to satiation show that the growth rate increases with temperature, but then declines again at high temperatures when the metabolic costs outweigh the somatic gains (Fig. 4) (Bjornsson et al., 2001; Bjornsson & Steinarsson, 2002). The temperature for maximum growth declines as fish increase in size, as does the growth rate. Fish of a particular size that are not fed to satiation have a lower maximum temperature. Light level (length of daylight) also affects growth, because G. morhua are visual feeders. The available evidence shows that under similar feeding conditions, the differences in growth rate between stocks are mainly due to temperature and light conditions and that genetically determined growth differences are small (Suthers & Sundby, 1993; Buckley et al., 2004; Jørstad et al., 2006; Karlsen et al., 2006; Taranger et al., 2006).

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Figure 3. Growth production and surplus production of Gadus morhua (Dutil & Brander, 2003): western Scotian Shelf (BF), east Baltic (EB), eastern Scotian Shelf (ES), Faroe Plateau (FP), Georges Bank (GB), Iceland (IC), north-east Arctic (NA), northern Grand Bank (NC), north Gulf of St Lawrence (NG), North Sea (NS), south Gulf of St Lawrence (SG), West Baltic (WB) and West Scotland (WS).

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image

Figure 4. Growth rates of four sizes of satiation-fed Gadus morhua at different temperatures. Four fitted masses of 100 (inline image), 250 (inline image), 1000 (inline image) and 5000 (inline image) g are shown. The dashed line connecting the curves is the locus of the maxima. Derived from Bjornsson et al. (2001).

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This combination of experimental and comparative work provides a consistent picture of the growth response of G. morhua as a species to temperature, which is useful when trying to predict the consequences of temperature change. The response will be greater at low temperatures (<c. 4° C) and will affect the smaller life stages more than adults. Unfortunately like most processes in biology this nice simple response function is subject to a number of ceteris paribus (other things being equal) clauses, which rarely are equal. In the case of growth these include the assumptions that their ambient temperature is known, that their food environment remains unchanged and that they do not have sophisticated behavioural responses to changes in the thermal environment (Lough et al., 1996; Buckley et al., 2004; Neat et al., 2006; Neat & Righton, 2007).

A rather simpler and more extreme example of a direct effect of environment on G. morhua comes from the Baltic Sea, where salinity may become so low that eggs sink into the bottom anoxic layers of the water column and sperm becomes immotile (Köster et al., 2005). Climate-induced changes in salinity (governed by the balance of inflow of saline water from the Skagerrak and precipitation and run-off in the Baltic basin) thus have a major effect on G. morhua recruitment. Salinity tolerance and buoyancy of eggs have been determined experimentally (Nissling & Westin, 1997) and this is one of the best examples of experimental work being directly useful in making predictions of climatic effects.

Indirect effects

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

Indirect effects of climate act by changing the trophic structure within which a species occurs and by increasing or diminishing the effects of predators, competitors, parasites and diseases. Thus the quantity and quality of food available may be altered, with consequences for growth, reproduction and mortality. The experimental and in situ results showing effects of temperature on G. morhua growth can be applied to this species at Iceland, where temperature in the upper 200 m has risen by >1° C since the early 1990s (Holliday et al., 2009). This has not resulted in the expected increase in growth rate, which has declined slightly (Fig. 5) for at least two reasons. The first is that a greater proportion of the G. morhua are now found in the colder northern parts of Icelandic waters and the second is that the principal prey species, capelin Mallotus villosus (Müller) have become much less abundant and have probably shifted their distribution to the north of Iceland (ICES, 2010b). It is also worth noting that the growth of G. morhua at Iceland would not be expected to respond strongly to temperature changes because the stock is in the middle of the thermal range for the species, where the response surface is fairly flat.

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Figure 5. Mass (M) of 5 year old Gadus morhua at Iceland from ICES (2010b). Inset is the 0–200 m temperature in south Icelandic waters from Holliday et al. (2009).

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Another example that illustrates the effects of quality and seasonality of food production comes from the North Sea, where the survival of G. morhua during their early life history can be related to the seasonal abundance and size composition of their copepod prey (Beaugrand et al., 2003). Changes in copepod species composition and seasonality can in turn be related to changes in regional climate (Beaugrand, 2003). There is evidence from the Baltic Sea that changes in fatty acid composition of plankton may be altering the seasonal timing of G. morhua spawning by affecting their maturation schedule (Tomkiewicz et al., 2009).

From individual to population

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

It has already been shown (Fig. 3) that there are considerable differences in surplus production among the North Atlantic G. morhua stocks due to differences in rates of growth and reproduction that are due in part to temperature (Dutil & Brander, 2003; Rätz & Lloret, 2003). The response of reproductive output to temperature is difficult to measure experimentally (Karlsen et al., 2006) and reproduction is generally dealt with at the population rather than the individual level (i.e. number of recruiting fish per unit of spawning stock biomass). Gadus morhua stocks at the cold end of the range (Greenland, Barents Sea) have better survival in early life when temperatures are relatively high and the reverse is true for the warm end of the range (Fig. 6).

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Figure 6. Recruitment for five Gadus morhua stocks illustrating the effect of temperature. The scales are ln (number of 1 year old fish), with the mean adjusted to zero. The Arcto-Norwegian and Icelandic stocks have been displaced vertically. Temperature is for the period when G. morhua larvae are pelagic. Stocks are: Greenland (inline image); Arcto Norwegian (inline image); Iceland (inline image); North Sea (inline image); Irish Sea (inline image).

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In addition to the observed differences in surplus production among different stocks, there are also changes over time within a stock due to changes in their physical and biological environment (Dutil & Brander, 2003). This means that a given stock may be able to produce more than it did previously, as occurred with gadoid stocks in the North Sea during the period from the mid-1960s to the mid-1980s (ICES, 1999) or it may decline at levels of fishing that had previously been sustainable. The decline of the Canadian G. morhua stocks was preceded by a period when growth rate, individual fish condition and reproductive output were reduced (Brander, 2007). There was probably a vicious positive feedback where decline in productivity and biomass was coupled with increased catchability to drive up fishing mortality and further reduce biomass (ICES, 2006; Mohn & Chouinard, 2007; Lilly et al., 2008).

Most fisheries assessment and management still rely too heavily on the assumption that the productivity of fish stocks does not change, which is the basis for the maximum sustainable yield (MSY) concept that is enshrined in all international declarations concerning sustainable fisheries, e.g. Johannesburg Declaration (UN, 2002). The idea of regime shifts in which productivity jumps from one steady state that lasts for a decade or more to another state has recently become popular, but evidence for underlying processes that would generate such a stepped series of steady states is poor and the alternative, that there is no steady state, should be taken much more seriously than it is currently. Regardless of the dynamics of the system, the important point is that far more attention must be paid to detecting changes in productivity quickly, in order to respond in time to avoid undesirable effects on fisheries and marine ecosystems. There are essentially two forms of management, precautionary and responsive, to deal with the risk posed by productivity changes. The precautionary approach dictates caution in the face of uncertainty (i.e. low levels of exploitation) and can be likened to driving slowly when conditions are poor. The responsive approach is to monitor the situation for signs of change or even to develop ways of predicting them and then implementing effective and rapid changes (e.g. rapid reduction in level of exploitation). There is undoubtedly a role for the detailed understanding of biological processes in designing cost-effective monitoring of productivity changes and this should become an area where fundamental biological knowledge becomes an integral part of the management of fisheries and marine ecosystems.

In relation to the productivity of G. morhua, some of the factors affecting growth have been dealt with, but there are problems with transferring experimental results in order to make predictions for populations. Measuring the actual conditions (temperature, salinity and food availability) that fishes experience in the sea is difficult. The new generations of data storage tags coupled with other forms of analysis [e.g. otolith microchemistry as used by Hüssy et al. (2009)] are providing a wealth of information that shows sophisticated behaviour in relation to depth and temperature such that simple predictions of the effects of climate change may be completely wrong (Metcalfe et al., 2009). Growth rates of fish reared in experimental systems and aquaculture are generally much higher than the rates observed in natural populations, leading some to the conclusion that fishes in nature are generally food-limited. The absurdity of this conclusion is evident if one applies the same reasoning to those of us who are not obese – does this make humans food-limited?

Prediction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References

Much scientific effort is now being devoted to detecting the effects of climate change on biological systems, understanding the processes and using this process information to make predictions of future effects (Keyl & Wolff, 2008; Rijnsdorp et al., 2009; Pörtner; 2010). With the current attention to climate there is a tendency to ascribe all changes to it, which can be dangerously misleading. The heading for a fairly sober report in the journal Nature on a discussion of climatic change in the North Sea (Schiermeier, 2004) read ‘Climate change lets fishermen off the hook’ and was eagerly seized on by some in the fishing industry to argue that controls on fishing were no longer needed. Unfortunately, the advent of new stresses on marine systems does not get rid of the old stresses caused by fishing, habitat degradation, pollution and so on. There are indeed quite clear interactions between climate and other pressures on marine populations. For example, heavily fished populations which have a reduced age range are more sensitive to environmental forcing (Ottersen et al., 2006).

The 20th century trajectory of G. morhua biomass in the Baltic Sea can be qualitatively explained by the interaction of four different pressures: predation by seals, eutrophication, climate-driven salinity change and fishing (Fig. 7) (Eero et al., in press). More detailed process studies can be cited to show how these affect natural mortality and survival during early life (Köster et al., 2005). A multispecies model with three trophic levels has been developed to show the interactions between fishing pressure and climate (salinity in this case) in the Baltic Sea (Lindegren et al., 2009). It incorporates uncertainty due to the model and to the climatic variables and projects the risk of extinction of the G. morhua stock during this century (Lindegren et al., 2010).

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Figure 7. Factors affecting spawning stock biomass of eastern Baltic Gadus morhua (inline image) from 1925 to 2006. The colours represent the influence (positive–negative) of different factors on G. morhua biomass. From Eero et al. (in press) reproduced with permission from the Ecological Society of America.

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Climate change has undoubtedly already had an effect on marine biota in the north-east Atlantic Ocean from plankton to marine mammals (ICES, 2008a); however, only a small part of these effects on distribution, abundance and phenology can be attributed to anthropogenic causes. There are large regional differences in the pattern of global warming and the area around the British Isles is at the extreme of warming, with increases of >1° C over the past 25 years (Fig. 8) ICES (2008b). This is many times greater than the global rate and is also much greater than that predicted for the region by global models (IPCC, 2007). There are large natural decadal temperature cycles in the North Atlantic Ocean and the regional warming is probably responsible for much of the observed trend since 1985 (Smith et al., 2007). This means that the region can be expected to show stronger evidence of climatic effects than elsewhere on the globe, but this is only partly due to anthropogenic climate change (ICES, 2008a). In addition, it would not be surprising if some of these regional trends go into reverse as the decadal cycle changes phase (Smith et al., 2007).

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Figure 8. Sea surface temperature (SST) of the north-east Atlantic Ocean shown as the mean for 2003–2007 minus the mean for 1978–1982. The plots are based on U.S. National Oceanic and Atmospheric Administration (NOAA) National Civil Defense Committee (NCDC) extended reconstructed sea surface temperature (ERSST) version 2, which is an extended reconstruction of global SST data based on international comprehensive ocean-atmosphere data set (ICOADS) (Worley et al. 2005) monthly summary trimmed group data (http://www.cdc.noaa.gov/).

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It would be wrong to leave the impression that the continuing upward trend in fishing mortality shown for Irish Sea G. morhua is representative of what is happening more widely, because this might be taken as evidence that fisheries management has failed to deal with the problem of overfishing. There is still a long way to go in securing sustainable fish populations and healthy marine ecosystems, but things are moving in the right direction. Fishing mortality has been greatly reduced in the north-east Atlantic Ocean over the past 10 years (Fig. 9) and the global record of fisheries management is improving (Worm et al., 2009; Grafton et al., 2010), although not before time.

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Figure 9. Standardized average fishing mortality (F) for 50 stocks of fish (ICES; http://ices.dk/datacentre/StdGraphDB.asp/).

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References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References
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Electronic References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Irish sea g. morhua: An example of a declining stock
  5. Direct effects on individuals and populations
  6. Indirect effects
  7. From individual to population
  8. Prediction
  9. References
  10. Electronic References