Background
Friday, 18 February 2011 15:13

The Pacific tuna fisheries are among the largest, most complex and valuable fisheries resources in the world.  In 2008, the most recent year with confirmed statistics, the total tuna catch exceeded 3 million tonnes (Williams and Terawasi, 2009; IATTC, 2009), comprised 70% of the total global tuna catch and was valued at USD 4.8 billion dollars. The Western and Central Pacific Fisheries Commission (WCPFC), the Inter-American Tropical Tuna Commission (IATTC) and the Western Pacific Regional Fisheries Management Council (WPFMC) are responsible for ensuring the sustainable use of this globally significant resource.

 

In managing these fisheries, these bodies are guided by their respective Conventions, and in particular, objectives within these to manage the catches to attain but not exceed the maximum sustainable yield (MSY). Conservation and Management Measures (e.g. WCPFC CMM 2008-01) and resolutions (e.g. IATTC Resolution C-09-01) are designed to ensure these primary objectives are met. 

In designing management strategies to meet these objectives, quantitative stock assessments and model based projections are used to allow consideration of the potential future responses of the tuna stocks to alternate management scenarios. While it is recognised that changing environmental processes can significantly influence population processes (recruitment, growth, natural mortality), current assessment models and projections rarely explicitly include such effects due to a lack of information on their impacts. There is now, however, conclusive scientific evidence that two significant and anthropogenically forced long-term shifts in global ocean conditions are currently in progress and will continue into the future, these being ocean warming (e.g. Barnett et al. 2005) and ocean acidification (Caldeira and Wickett, 2003; Feely et al. 2004). Both temperature and pH are environmental factors that can influence the growth, condition and survival of fish (e.g. Buckley et al. 1990; Ishimatsu et al. 2004) and therefore affect population dynamics and abundance (Pepin, 1991).

The ecosystem model SEAPODYM is structured to evaluate the impact of such environmental changes (Lehodey et al 2008).  It explicitly integrates physical oceanography with forage and tuna population dynamics to spatially predict the distribution and abundance of tuna (Lehodey et al 2008).  The model is currently being used to forecast the changes in tuna distribution and biomass associated with the current IPCC scenarios for ocean warming in the Pacific Ocean (Lehodey et al, 2010).

Unfortunately, no consideration has been given to the potential impacts of ocean acidification upon the target tuna populations in the Pacific, despite growing evidence that it is likely to have a very profound impact upon many marine species populations and ecosystems by the end of this century and may threaten some of the world’s most productive fisheries (Guinotte and Fabry, 2008).  SEAPODYM could be applied to also forecast the impacts of ocean acidification on tuna resources in the Pacific if information on the effects of pH on population dynamics of tuna can be collected.

 

Oceanic waters show both spatial and temporal variation in pH, with surface waters being slightly alkaline (average pH 8.2) and varying by about 0.3 units due to physical and biological oceanographic processes (Raven et al., 2005). A number of factors influence oceanic uptake or release of CO2 from surface waters, including atmospheric CO2 concentrations, sea surface temperature, biological (primary) production, and physical oceanographic processes such as upwelling of CO2 rich waters.

Ocean acidification is defined as the “change in ocean chemistry driven by the oceanic uptake of chemical inputs to the atmosphere, including carbon, nitrogen, and sulfur compounds” (Guinotte and Fabry, 2008). Anthropogenic atmospheric CO2 is by far the largest contributor to ocean acidification today, with an estimated 30-50% of global anthropogenic CO2 emissions absorbed by the world’s oceans (Feely et al 2004, Sabine et al. 2004, Orr et al 2005). The net effect of adding CO2 to seawater is to increase concentrations of H2CO3 (carbonic acid), HCO3- (bicarbonate ion) and H+ (hydrogen ions) and decrease concentrations of CO32- (carbonate ions) and lower pH (Fabry et al 2008). Hence, the increased CO2 absorption has led to a 30% increase in oceanic hydrogen ion (H+) concentration and a reduction in pH (increased ocean acidity) by 0.1 units since the start of the industrial revolution (Caldeira and Wickett, 2003) as well as a reduction in the number of carbonate ions (CO2-3) available.  It is estimated that this future uptake of atmospheric CO2 by global oceans will further reduce the pH of oceanic surface waters by an average 0.3-0.4 units by 2100 (Caldeira and Wicket, 2003, 2005), with larger reductions predicted at higher latitudes (Ilyina et al. 2009b). These reductions in the pH of ocean surface waters represent shifts in pH larger than any thought to have occurred in millions of years, outside of rare catastrophic events  (Feely et al. 2004). The predicted rate of change is very fast relative to past rates of change.

Changes in water chemistry associated with increased CO2 are expected to impact upon tuna populations, and this sensitivity is expected to vary depending on the specific developmental stage.  For productive species like tuna, the rate of survival typically increases as fish develop and mature, and the environmental processes that significantly affect recruitment numbers act primarily upon the early life history stages (Pepin, 1991; Ferron and Leggett, 1994). Only a small proportion of larvae survive through to adulthood, with the consequence that even a tiny change in the mortality rate during these stages can have orders of magnitude greater effect on the number of recruits added to the adult population (Pepin, 1991; Buckley et al., 1999). Small changes have large implications for fisheries and their ability to gain maximum yields from stock while not compromising the reproductive capacity of the populations.

In the only known relevant experiment involving tuna, Kikkawa and colleagues (2003) found that 100% mortality of eggs of Eastern little tuna (Euthynnus affinis) occurred within 24 hours of exposure to elevated pCO2 levels.  However, the levels tested far exceeded those predicted using IPCC scenarios and these results cannot be used to confidently imply responses under projected ocean acidification levels.

However, concern over the potential impacts of ocean acidification on tuna populations arises from the rapidly growing body of evidence indicating significant negative effects on the physiology, growth and survival of a diverse range of other fish and marine organisms, in response to elevated CO2 levels (Fabry et al. 2008; Guinotte and Fabry, 2008; Raven et al., 2005). There are two main mechanisms by which elevated pCO2 and associated changes in ocean chemistry (including decreased pH) adversely impact marine organisms, these being via decreased carbonate saturation state, which directly affects calcification rates, and via disturbance to acid–base physiology (Fabry et al. 2008; Guinotte and Fabry, 2008). Indirect impacts upon a given organism or population are also expected to occur due to direct impacts upon other species within the ecosystems they are part of.

The reduction in the availability of carbonate ions (reduced carbonate saturation) due to increased CO2 and lowered pH (Feeley et al. 2004) makes calcification processes much more difficult for organisms which build skeletons, shells or other structures out of CaCO3, and in undersaturated waters, dissolution of CaCO3 is favoured (Guinotte and Fabry, 2008). Decreased calcification in response to the  reduced carbonate saturation state has been demonstrated in numerous species of coral, coccolithophores, molluscs, echinoderms and other calcifying organisms (e.g. Raven et al., 2005).

However, little attention has been given to the potential impacts on the development/growth of fish otoliths, which play a critical role in fish balance and hearing, and which are in large part comprised of aragonite, a form of calcium carbonate. The only known study, that of Checkley et al (2009), surprisingly found that larval white sea bass, grown under elevated CO2 conditions, produced significantly larger otoliths compared to fish of the same size/age grown under control conditions. However, the impact of abnormally large otoliths (for size) upon fish condition and survival is still unknown (Checkley et al., 2009) and the impact of ocean acidification on otolith formation may depend on species specific capacity for acid-base regulation in the tissues surrounding the otoliths (Fabry et al 2008). Given the importance of otoliths to fish movement and behaviour determining potential impacts on tuna is a priority.

Acid base metabolism is another key area of potential impact in tuna. Increased partial pressure of CO2 in the blood (hypercapnia) due to increased environmental pCO2, can induce acidosis in body tissues and fluids of larger marine organisms including fish (Portner et al., 2004), with evidence that this can impact on numerous processes including the oxygen carrying capacity of blood (Portner and Reipschlager, 1996), protein synthesis and growth (Langerbuch and Portner, 2003) and reproduction (Portner et al, 2004). Such impacts are also likely to vary between developmental stages.

Unfortunately, studies on fish species which attempt to examine the likely impacts of ocean acidification on fish metabolism are relatively few. Many studies have tested for the potential effects of CO2 levels which are much higher than projected under IPCC scenarios (Fabry et al 2008). Those studies have typically found very high mortality levels (Kikkawa et al., 2006; Hayashi et al. 2004), including in early developmental stages (eggs, larva) (Kikkawa et al. 2003, Ishimatsu et al. 2005) and other effects including reduced cardiac output (Ishimatsu et al. 2004) and metabolic capacity (Michaelidis et al. 2007). However, for such information to be relevant to the management of fisheries, they clearly need to be conducted through simulating the ocean chemistry conditions (CO2 and pH levels) that are reflective of those predicted using the IPCC scenarios. Only very recently has research been presented (to the International Symposium on “Climate Change Effects on Fish and Fisheries: Forecasting Impacts, Assessing Ecosystem Responses, and Evaluating Management Strategies”, 25-29 April 2010) on ocean acidification impacts on larval fish using IPCC projections and these results are as yet unpublished in scientific journals, but suggest that susceptibility to ocean acidification will be species specific (Hollowed et al, 2010 and symposium abstracts therein). 

Evidence has also been found for impacts of ocean acidification upon processes critical to reproductive success in marine organisms, namely gamete viability, fertilisation rates and embryonic development. Havenhand and colleagues (2008) tested the response of sea urchin gametes and larvae to seawater with CO2 reduced pH by -0.4, (the upper limit of projected change by 2100) and found statistically significant reductions in sperm swimming speed and percent sperm motility and fertilisation rates (down 24%) and subsequent embryonic development. This study emphasises the importance of using CO2 and pH levels predicted using IPCC scenarios and its findings suggest that it will be critically important to examine gamete and fertilisation sensitivities in other broadcast spawning marine species (including tuna) for which sperm are exposed to environmental pH prior to egg fertilisation. Similar effects on sperm activation and motility were found for white sturgeon (Ingerman et al. 2002), further emphasising the potential effects in fish. Similar effects on fertilisation and sperm motility have been found in other studies of sea urchin (Kurihara et al 2004).

Many environmental factors are thought to contribute to recruitment variation including water temperature (e.g. Buckley et al 1990), food availability/starvation (e.g. Lett and Kohler, 1976) and predation (e.g. Leggett and Deblois, 1994). The synergistic impacts of pH in combination with other environmental stressors likely to be encountered in the future (increased temperature) and encountered normally (starvation) may better define the likely direct impacts of ocean acidification upon tuna recruitment.

Recognition of and concern over the potential for recent, rapid changes in ocean chemistry to severely affect marine organisms, food webs, biodiversity, and fisheries within the next 50 years, led to the signing of the Monaco Declaration (2008) by 155 world leading ocean acidification researchers from 26 countries. The declaration states that ocean acidification is underway, is accelerating and severe damages to marine ecosystems are imminent and could “affect marine food webs and lead to substantial changes in commercial fish stocks, threatening protein supply and food security for millions of people as well as the multi-billion dollar fishing industry”.

This international concern is now also being reflected in government policy. The US Congress requested that the National Research Council conduct a study on ocean acidification in the Magnuson-Stevens Fishery Conservation and Management Reauthorization Act of 2006 to assist federal agencies develop their understanding of and ability to address the issue. Shortly after the study was underway, Congress passed another law—the Federal Ocean Acidification Research and Monitoring (FOARAM) Act of 2009—which calls for, among other things, the establishment of a federal ocean acidification program (NRC, 2010). This report  “Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean” was presented to a US Senate Committee Hearing on April 22nd 2010, with the report concluding that “Present knowledge is insufficient to guide federal and state agencies in evaluating potential impacts for management purposes” and identifying an urgent need for Federally funded research into the impacts of ocean acidification, specifically focusing on 8 key areas including: Understand the physiological mechanisms of biological responses; Investigate the response of individuals, populations, and communities; Understand ecosystem-level consequences; Investigate the interactive effects of multiple stressors; Understand the socioeconomic impacts and informing decisions.

The unaccounted impacts of ocean acidification (and warming) upon tuna stocks in the Pacific (and globally) represent a serious risk to the achievement of sustainability based management objectives for both Regional Fisheries Management Organisations and for the policies of sovereign states responsible for fisheries management in the Pacific region.  The higher risk expected for early life history stages and the observations from other marine species suggest that initial efforts to elucidate the impacts of projected ocean acidification upon tuna populations should focus on processes and stages that are critical to recruitment success: gamete impacts, fertilisation rates, embryonic development, hatching rates, condition, development, growth and survival in pre and post feeding larvae.  To understand the implications for tuna population dynamics and for tuna fisheries, these potential effects need to be tested and taken into account in the population models currently used to evaluate stock dynamics in the Pacific Ocean. Yellowfin tuna has been chosen as the initial subject of these investigations, given that the required facilities and expertise to conduct these experiments for yellowfin tuna already exist and it is a key target species in both the WCPO and EPO.

 


 
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