Figure 1. Mean weight at age of Alaska female halibut by decade.
Executive Summary
We propose to undertake a three year investigation of widely observed interdecadal changes in the growth and recruitment of Pacific halibut. Recent advances in our understanding of climatic variability have led to a general theory of 20-30 year climatic regimes across the North Pacific. The remarkable temporal coincidence of changes in the general atmospheric and oceanic circulation with changes in halibut population characteristics provide a conceptual framework from which to organize the investigation. The nature of the work ranges from retrospective analysis in the first year to modelling and process oriented field studies in the second and third years. A well qualified fisheries oceanographer (Ph. D. or possibly doctoral candidate) is required to conduct the studies. Cost of the study is dependent primarily on the researcher, but should be in the range US$100,000 - US$180,000.
INTRODUCTION
During this century there have been dramatic and persistent changes in the growth and recruitment of Pacific halibut that cannot be readily explained by changes in stock size (Clark 1995). The staff has taken account of these medium-term changes in recommending an optimal exploitation rate, but we have not known the reason (or reasons) for them.
Recent climatological research has detected the occurrence of abrupt changes in North Pacific atmospheric and oceanic circulation, the most recent event occurring in the mid-1970s. It is increasingly apparent that the entire biota of the North Pacific, including halibut, underwent a major change beginning around that time. Similar regime shifts, detectable from various data series, occurred earlier in the century.
The staff is proposing a research project to relate the biological changes observed in the stock to the physical changes observed in the ocean. The aim is to identify distinct climatic regimes and the behavior of the stock under each. The benefits would be a better understanding of how the environment affects the stock and possibly a small increase in long-term yield.
The structure of this proposal is as follows. We first detail the aforementioned climatic and biological changes. We then summarize previous work on halibut-environmental interaction and their associated hypotheses. From the older work and newer ideas, a set of hypotheses are established around which the proposed research will focus. The investigative and analytical methods required to investigate the hypotheses are described. Finally, a set of projects are then proposed along with expected personnel, time and financial requirements.
BIOLOGICAL CHANGES IN THE HALIBUT STOCK DURING THIS CENTURY
Over the last fifteen years the growth of halibut has decreased dramatically, especially in Alaska. An eleven-year-old female landed in Kodiak was a 40 lb fish in 1980. Now it is less than 20 lb (Figure 1). Fifteen years ago fish of a given age were substantially larger in Alaska than in British Columbia; now there is no difference. In both respects, halibut growth is similar to what was observed in the 1920's and 1930's. An increase occurred sometime during the 1940's, and the present decrease began in the mid-1970's. Fish are also maturing at a smaller size now than they used to (Figure 2), while the age at maturity is quite close to what it has always been.
There have been clear decadal variations in halibut recruitment all through the century, or at least since about 1935 (Figure 3). Most recently we saw a run of good year-classes spawned in the late 1970's and early 1980's, apparently followed by a run of poor year-classes. This kind of alternation has sometimes been viewed as a cycle, but could just as well reflect distinct periods of different environmental conditions.
Additional discussion of biological changes in the halibut stock is included in the literature review section below along with the various hypotheses advanced as explanations for the changes.
CLIMATE VARIABILITY IN THE NORTH PACIFIC
The climate of the North Pacific is driven by the location and intensity of seasonally varying atmospheric pressure cells. During the most climatologically active months of November to March, the Aleutian Low pressure system covers much of the North Pacific, while the Subtropical High pressure system is most active in the summer months. Each of these systems can cover an area of several million square kilometers. In addition to creating conditions that establish seasonal weather patterns, these atmospheric systems affect oceanic conditions via changes in vertical and horizontal flow driven by surface wind stress. Examples of wind-driven flow changes include redirection of surface currents, mixed layer depth turnover, and enhanced or suppressed coastal upwelling. These processes in turn affect biological primary production and, ultimately, upper-level trophic species in an ecosystem driven by changes at the lowest production level working their way up to the top level predators.
Recent convincing evidence has accumulated that the climate of the North Pacific, and in particular the activity of the Aleutian Low pressure system, has changed markedly from 20 years ago (Trenberth 1990, Ebbesmeyer et al. 1991, Graham 1994, Figure 4). Since the winter of 1976/77, winters in the North Pacific have generally been marked by intense, large-scale Aleutian Low events (Trenberth and Hurrell 1994). The center of the Low has deepened (i.e. lower central pressure) and shifted eastward by several hundred kilometers. The physical impacts of this change in behavior are numerous, but include the following: warmer air and sea surface temperatures in Alaska and Alaskan waters (McClain 1984, Rogers 1984), more frequent and severe storm activity (Salmon 1992), increased vertical advection (upwelling) (Miller et al. 1994) and decreased mixed layer depth (Polovina et al. 1995) across most of the Gulf of Alaska.
To set the stage for the mechanistic model relating climate variability with biological changes in the halibut stock, it is useful to briefly diagram the current oceanographic model of climate driven surface flow in the North Pacific. The model (Figure 5) was initially proposed by Chelton and Davis (1982), and extended by Hollowed and Wooster (1992) and Francis and Hare (1994). In this model, the circulation of the North Pacific is driven by variations in the strength of the wintertime Aleutian Low. The major circulatory feature of the north Pacific is the presence of two permanent oceanic gyres - the cyclonic Subarctic (also called the Alaska) gyre and the Subtropical gyre. These two gyres are fed by the eastward flowing Subarctic Current via its two coastal extensions, the northward flowing Alaska Current and the southward flowing California Current. The key aspect of the model is the "out of phase", or inverse, relationship between the two gyres. In one state (Hollowed and Wooster's Type "B"), the Aleutian Low is intensified resulting in a spinup of the Subarctic Gyre and enhanced flow into the Alaska Current. In the other state ("Type A"), featuring a weakened Aleutian Low, the California Current is strengthened at the expense of a weakened Alaska Current. A summary of the major atmospheric and oceanic conditions in the North Pacific under each state is provided in Table 1. In what is termed the "regime shift" aspect of this general model, the circulation of the north Pacific changed from Type A to a Type B pattern in the winter of 1976/77 and has essentially remained locked in a Type B pattern since.
Several investigators have speculated that the 1976/77 regime shift is but the most recent in a succession of events (Francis and Hare 1994, Hare and Francis 1995, Ware 1995, Francis et al in review, Mantua et al. in review). In the 20th century, it has been proposed that four distinct climatic regimes have occurred (Figure 6). The regimes have averaged 25-30 years in duration, with the transitions taking place in the mid-1920s, mid-1940s and mid-1970s. The relationship of these regimes to the El Nino Southern Oscillation (ENSO) phenomenon is unclear. Several simulation and observational studies have shown that much of the climatic variability in the North Pacific is independent of ENSO (Latif and Barnett 1994, Zhang et al. 1996, Mantua et al. in review). Other investigators, however, maintain that ENSO plays a dominant role in the North Pacific (Kumar et al. 1994, Trenberth and Hurrell 1994). This remains an extremely active research area with important ramifications for biological research on halibut.
GENERAL BIOLOGICAL EFFECTS OF REGIME SHIFTS
Biological changes associated (or at least co-incident) with these climatic regime shifts have been spectacular. Evidence of bio-physical connections are evident throughout the North Pacific and Bering Sea. Since the 1976/77 regime shift, sharp increases have been noted in primary (Venrick et al. 1987) and secondary (zooplankton) (McFarlane and Beamish 1992, Brodeur and Ware 1992, Brodeur et al. 1996) production, as well as in salmon (Hare and Francis 1995) and many groundfish species (Beamish 1993). Concurrently, other populations including king crab (Blau 1986), shrimp (Anderson 1991), Steller sea lions (Springer 1992) and several species of marine birds (Piatt and Anderson, in press), declined sharply with the onset of the new regime. A dramatic decline in average size of Pacific halibut across much of its range also began following the winter of 1976/77 (Clark 1995). These differences have been noted in populations ranging from southern California to the Hawaiian Islands to the Bering Sea.
A very close linkage has been established between changes in oceanic
conditions driven by Aleutian Low variability and Alaskan salmon production
(Hare and Francis 1992, Beamish and Bouillon 1993, Francis and Hare 1994,
Hare and Francis 1995, Hare 1996). The historical variability in catches
of the major Alaskan salmon stocks and the proposed regime changes in production
levels is illustrated in Figure 7. In fact, the most
advanced (though still speculative) bio-physical model linking climate
and fish population response has been advanced for Alaskan salmon. Recent
research has pinpointed the first year of marine residence as the most
likely period of influence (Pearcy 1992, Francis and Hare 1994). Increased
marine growth resulting from elevated zooplankton production within the
Alaska Gyre is hypothesized to have driven the production increase (Hare
1996).
PROPOSED RESEARCH RELATING TO HALIBUT
It seems clear that the halibut stock, along with other fish stocks, has been affected in important ways by climate change in this century. With global warming almost surely on the way, the next century is likely to be even more eventful.
At present we do not know the actual mechanisms by which climate change affects halibut growth, maturation, and recruitment. Nor can we distinguish the effect of physical factors, such as ocean currents and temperature, from biological factors such as the abundance of prey species, other fish species, and halibut themselves. We can and do manage the stock by allowing for these medium-term changes without knowing their causes, but there is no doubt that we could improve our models, our biomass estimates, and our forecasts if we had a better understanding of the natural processes that clearly have a large effect on the productivity of the stock.
The Halibut Commission does not have the wherewithal to launch a research program of its own in fishery oceanography, but we have the opportunity to capitalize on the international research programs now in progress by engaging an expert to apply recent research results to Pacific halibut and to collaborate with other researchers in the design of large-scale studies. Moreover, it is very appropriate for the Commission to be active at least to this extent, because it is the best way to assure that the international effort will address questions about halibut, and because we have some of the longest data series to contribute to the international effort.
LITERATURE REVIEW AND SUMMARY OF EXTANT HYPOTHESES
One of the most fundamental questions in fisheries science revolves around the relative importance of natural and fishing-induced changes in the characteristics, e.g., production, growth, recruitment and fecundity of a fish population. A related set of interesting questions concerns the relative roles of density-dependent (ecological) and density-independent (environmental) control of the same characteristics. A number of studies have addressed these issues as related to halibut. As will be clear from the following brief review, few clear answers have been found. A number of hypotheses have been generated, however, and investigation of the hypotheses would be a valuable first step in addressing potential new angles on the environment-halibut relationship.
The Thompson-Burkenroad debate
The Thompson-Burkenroad debate, on the decline and subsequent increase in the Pacific halibut population between 1920 and 1940, is perhaps the best known dispute in fisheries. The debate was thoroughly reviewed by Skud (1975) so only a cursory description of the debate is included here. Thompson (1950, 1952) took the view that the changes in abundance could be solely ascribed to fishing pressure, a view first expressed in Thompson et al. (1931). This view was strongly challenged by Burkenroad (1948, 1950, 1951, 1953) in a series of articles, in which he ascribed much of the variability to climatic forcing. The debate was joined by supporters on both sides. On the side of Burkenroad were Huntsman (1953), Ketchen (1956) and Fukuda (1962). Bell and Pruter (1958), in support of Thompson, concluded that "The hypothesis that fishing, not natural forces, has been the major factor in affecting the stocks appears well founded." Dickie (1973) criticized the scientific community for polarizing between the two camps, but reserved special criticism for Bell and Pruter (1958), whose conclusions he characterized as "deceptively simple." Since Skud's review, the state of affairs has remained essentially unchanged. That is, there is still no clear consensus on what drives variability in Pacific halibut production, despite what is "arguably the best fisheries dataset in the world" (Hilborn and Walters 1992).
Historical variation in halibut growth rates
The growth of halibut has varied dramatically during the 20th century. The change in size at age of halibut was well known by the early 1950s (IPHC 1954) and extensively analyzed by Southward (1962, 1967). Using a back-calculation method on otoliths collected as early as 1914, Southward concluded that halibut growth rates had shown three distinct growth regimes between 1900 and 1965. Growth was higher than the long term mean from the early 1900s-mid 1920s and again from the mid-1950s-mid 1960s, and lower than the long term mean in the intermediate period. Schmitt and Skud (1978) also found long term changes in fecundity at age, doubtless related to the changes in size at age. In apparent contrast to the findings cited above, McCaughran (1981, 1987), in an analysis of mark-recapture data, found no temporal changes in growth rates. However, as noted in Trumble et al. (1993), the two sets of studies examined different age ranges of fish, with McCaughran restricting his analysis to fish greater than 10 years of age when tagged. The apparent conclusion to be drawn is that growth of young halibut has changed markedly over time, whereas annual growth of adults has remained relatively constant.
Subsequent to the pioneering work of Southward (1962), the methodology of calculating length at age on the basis of otolith increments has come under scrutiny. A neccessary requirement for back-calculation to be valid, at least for the method employed by Southward, is that otolith growth and body growth remain proportional. It has been established for a number of species, however, that otolith and body size can become "uncoupled", suggesting that the two growth rates are independent (Secor and Dean 1989, Mosegaard et al. 1988, Reznick et al. 1989). For Pacific halibut, Clark (1992) has convincingly demonstrated that the otolith-body size relationship has changed over time and, therefore, the quantitative (though not necessarily the qualitative) results of Southward (1962, 1967) are invalid. During the 1990s, however, a great deal of theoretical and empirical work has been done on back calculation and there appear to be promising avenues to explore. Because the IPHC has such lengthy time series collections of otoliths, it is highly worthwhile that all attempts are made to establish the utility of back calculation techniques. A summary of the current and proposed methods is given in the section below on research approach and methodology
Stock-recruitment, density dependent growth and survival
Several studies have attempted to advance the hypothesis that stock recruitment and/or halibut growth is density dependent. Schmitt and Skud (1978) and Deriso (1985) both presented analyses they said suggested density dependent production. The latter study was in contradiction to a pair of earlier IPHC studies (Quinn 1981 and Deriso and Quinn 1983) in which the hypothesis of density independent production was supported. The change in conclusion derived from the addition of nine years of data. In one of the more extensive examinations of factors affecting recruitment, Parker (1989) argued that reproductive success was dependent upon favorable wind-driven currents retaining pelagic larvae over the continental shelf. However, he accepted that density dependence was a critical factor and included an index of biomass in all his models. Several investigators have pointed out that the pattern of observed recruitment of eight year old halibut follows a strikingly regular cyclic pattern. Parker et al. (1995) showed that the index of recruitment had an almost identical period as the 18.6 "lunar nodal" cycle. They were unable to provide a mechanism whereby such a minor physical effect could produce such pronounced changes, suggesting that perhaps some sort of systemic amplification was at work. In a review of flatfish stock-recruitment relationships, Iles (1994) concluded that "For Pacific halibut it has not proved possible to identify an admissible stock-recruitment relationship".
In an innovative study, Hagen and Quinn (1991) analyzed otolith growth during the first 5 years of life from 26 year classes to identify patterns of annual growth and the sources of temporal variation. By developing a linear model, they were able to partition annual growth into year and year-class effects. To the extent that otolith growth reflects body growth, they concluded that annual halibut growth is determined by environmental conditions and not density dependence. However, they found that year effects on growth steadily declined over the 5 growth years, while the year-class effect steadily increased, indicating that density-dependent effects might be setting in at the later juvenile stage.
Halibut growth and production in relation to temperature and environmental factors
Temperature is commonly cited as an important environmental influence, particularly on younger fish. There appear to be few studies examining the relationship between temperature and halibut. The earliest literature reference at hand is Ketchen (1956) who looked at air temperatures and halibut landings. Ketchen found a "good correlation" between Masset, British Columbia air temperatures and lagged commercial fishery CPUE in two areas:
In the Hagen and Quinn (1991) study described above, a strong linear relationship was found between interannual SST and otolith growth from ages 0 to 2 years. The relationship was positive, i.e., warmer temperatures resulted in faster growth, and remained so up to age 4, though not as strongly. They attributed the decreased temperature effect to the emigration of older juveniles from nursery areas and thus away from relatively constant ambient temperatures. In the Parker (1989) study, temperature was - rather surprisingly - not included among the various factors analyzed.
One common theme in almost all the studies cited above is the call to continue research on these issues. Even Bell and Pruter (1958), the most vocal critics of previous environmental-fisheries studies urged that studies of the relative roles of physical and biotic factors "have been a matter of continuous attention and must remain the object of a very significant share of the research funds available for the study of the Pacific halibut." At the conclusion of their study, Hagen and Quinn (1991) urged further analysis of otolith growth as a means of further investigating several questions. For instance, the aforementioned low-frequency variability in SST may be linked to the interdecadal variability in juvenile growth rates. And they believe that by extending the otolith growth record back in time, it may be possible to delineate the route by which compensatory mechanisms regulate halibut population levels (if in fact they do) and to separate the possible confounding effects of the environment.
TOP LEVEL HYPOTHESES ON WHICH TO FOCUS RESEARCH
To focus the proposed research on climate change and halibut biology, it is useful to work within a conceptual framework to shape questions that can be addressed. This framework can consist of central questions of interest and specific testable hypotheses.
Central questions of interest
1b. If so, do growth and recruitment vary similarly?
1c. Further, do the trends resemble any of the several proposed climatic models (Francis and Hare 1994, Parker et al. 1995, Ware 1995)?
2b. What causes the rare exceptional year class (e.g., 1977, 1987)?
7a. What are the prospects of deriving environmental indices from the otolith archive, as has been done with corals, tree rings, etc?
Hypotheses
H1o - Year class strength of halibut is determined primarily in the first years of life and is affected by interannual and interdecadal changes in climatic forcing.
H2o - Rapid growth in the early juvenile phase results in large year classes.
H3o - Variation in size at age is determined largely by interdecadal changes in physical conditions.
H3a - Size at age is a purely density dependent process
H4o - Variation in recruitment is determined largely by interdecadal changes in physical conditions.
H4a - Recruitment is a purely density dependent process (i.e., determined by stock size)
H5o - Gulf of Alaska and Bering Sea bottom temperatures - where halibut reside - show the same response to atmospheric processes as do sea surface temperatures.
RESEARCH APPROACH AND METHODOLOGY
By its very nature, the work called for to conduct this project is interdisciplinary, drawing on the fields of oceanography, ecology, atmospheric science and fisheries. In addition, a strong background in statistical methodology is likely to be required as well as some modelling skills. While interdisciplinary research is sometimes criticized from a disciplinary camp, understanding of complex issues is likely to derive only from an integrative approach. While disciplinary investigations provide a great deal of information, the real world is not made up additively of the elements into which it has been divided by academic disciplines (Daly and Cobb 1989).
There are two fundamental approaches to science - the experimental/predictive and the historical/comparative. Each is concerned with developing an understanding of order in the natural world. The approaches, however, are fundamentally different. The former seeks to break down the world into finer and finer detail, trying to reduce system processes to their "fundamental behaviors" (Francis and Hare 1994). Historical science, however, arises out of the concept of contingency, i.e., that the observed state was arrived at from a sequence of antecedent states. The final state is therefore dependent, or contingent, on everything that preceded it. The historical/comparative method of science allows us to investigate processes that are non-replicable. It is a three step circular process, proceeding from observation to model development/refinement to model testing with historical data.
In order to successfully carry out this project, the investigator will require a substantial quantitative/statistical background. Historical and comparative science analyses generally involve multivariate and/or time series statistics. The multivariate methods, e.g., factor analysis, canonical correlation, discriminant analysis, are useful to identifying and documenting patterns in datasets; time series analysis is critical to quantifying historical variation in time sereis of interest. Other recommended statistical tools might include superposed epoch analysis (see Prager and Hoenig 1989 for a fisheries application) to investigate extreme recruitment events, randomization tests such as applied in the Hagen and Quinn (1991) analysis and generalized linear and additive models to rank environmental effects.
As noted earlier, back calculation of fish size based on measurement of body parts (otoliths in the case of halibut) is not the straightforward technique it was believed to be up until the late 1980's. After a number of studies demonstrated uncoupling of fish and otolith growth, the back calculation technique was declared inappropriate in mnay situations (Francis 1990, Francis 1995). Recently, however, Campana (1990) has devised a "Biological Intercept" backcalculation technique that appears to circumvent many, if not all, of the deficiencies of the earlier methods. It is essential that the investigator of this project pursue this thread as improvements to the technique continue and studies based on the Biological Intercept mehtod enter the literature (e.g., Secor and Dean 1992, Campana 1996). An excellent summary on otolith analysis is Stevenson and Campana (1992) and the Campana and Jones (1992) paper contained in that volume, which details the Biological Intercept method. Two other recent techniques also show promise in the area of otolith analysis and might fruitfully be considered for this project: the "Temporal Signature Technique" (Ogle et al. 1994) and Hatch Date Analysis (Methot 1983).
Perhaps the most important qualification for this project is that the
investigator have a solid oceanography background. The goal of this project
to understand how the physical environment affects halibut biology and
a more than cursory understanding of how the ocean responds to climate
variability over different time scales is critical. Further, the emphasis,
at least initially, is on the longer term (decadal to interdecadal) dynamics.
It would be beneficial to establish ties with PICES (the North Pacific
Marine Science Organization) which has taken the forefront in exploring
decadal scale climate variablity in the North Pacific. PICES has established
a dozen working groups on North Pacific physics and biology and input/participation
by a member of the IPHC would be welcome. The references cited cited in
this proposal provide a good entry to the Climate Change literature and
PICES has published several volumes with relevant papers. Finally, there
are several climate change-fisheries research groups around the Pacific
Rim with whom collaboration should prove useful. Such research groups can
be found at the Univ. of Washington, Univ. of British Columbia, the National
Marine Fisheries Service and in collaborations such as the PICES-GLOBEC
Climate Change and Carrying Capacity, NMFS-led Ocean Carrying Capacity
study, Southeast Bering Sea Carrying Capacity and Fisheries Oceanography
Coordinated Investigations.
RESEARCH PROJECTS, TIMELINE AND EXPECTED RESULTS
The first year of work carried out in this project can be spelled out in much more detail than work planned for the second and third years. This is mostly due to the fact that conclusions drawn from the first year will form the basis of where to continue investigations. It also allows the investigator flexibility in adapting the project to his/her own strengths and interests.
Year 1
RESEARCH BUDGET
It is recommended that three years funding be committed to this project. A shorter time frame, while more economically prudent, would be unlikely to result in significant achievements. The single most important factor for this project is the person recruited to carry out the research. With a three year window, the project could be setup in either of two ways.
|
Item |
Low cost |
High cost |
Years required |
Low total |
High Total |
| Salary | 20,000 | 50,000 | 3 | 60,000 | 150,000 |
| Computer | 0 | 5,000 | 1 | 0 | 5,000 |
| Software | 0 | 1,000 | 3 | 0 | 3,000 |
| Travel | 1,000 | 3,000 | 3 | 3,000 | 9,000 |
| Otolith work | 0 | 3,000 | 1 | 0 | 3,000 |
| Field work | 0 | 6,000 | 2 | 0 | 12,000 |
| Total | 63,000 | 182,000 |
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Figure 1. Mean weight at age of Alaska female halibut by decade.
Figure 2. Proportion mature at length of Alaska female halibut during
two distinct regimes.
Figure 3. Spawning biomass and subsequent recruitment of 8 year old
halibut. Points on the plot
for the 1977-88 brood years are distinctly outside the clous of points
for previous brood years.
Figure 4. The winter atmospheric and spring oceanic effects of the
1976/77 climate regime shift.
Units are mb for SLP and °C for SST. (Reproduced from Hare 1996).
Figure 5. Diagram of circulation patterns associated with Type-A and
Type-B
ocean conditions (Reproduced from Hollowed and Wooster 1992).
Figure 6. Two indicators of large-scale, long-term climate variability
over the North Pacific in the
20th century. The time series show the polarity of the Pacific
Decadal Oscillation as reflected in
winter SST and SLP, along with intervention model fits (Reproduced
from Mantua et al. in review).
Figure 7. Time history (dashed lines), intervention model fits (thin
solid lines)
and estimated interventions (thick solid lines) for Alaska salmon time
series
(from Francis and Hare 1994).