III. P lan kton com m unities
ICES Marine Science Symposia, 219: 169-181. 2003.
Biological response in a changing ocean environment in Newfoundland waters during the latter decades of the 1900s Eugene B. C o lbou rne and John T. A nderson
Colbourne, E. B., and Anderson, J. T. 2003. Biological response in a changing ocean environment in Newfoundland waters during the latter decades o f the 1900s. - ICES Marine Science Symposia, 219: 169-181. Ocean temperatures on the Newfoundland Shelf during the past several decades have experienced near-decadal oscillations superimposed on a general downward trend. In particular, the decade o f the 1990s has experienced some of the most significant varia tions since measurements began during the mid-1940s. Ocean temperatures, for example, have ranged from record low values during 1991 to record highs during 1999 in many areas, particularly on the G rand Bank o f Newfoundland. Coincident with the trends in ocean climate many commercial fish species have shown changes in abun dance, particularly during the decade of the 1990s. Recruitment in Newfoundland cod stocks, for example, has declined almost steadily since the 1960s, reaching historical low values by the early 1990s. During the cold early 1990s, with fishing m oratoria in place, recruitment continued to decline. However, by 1995, ocean temperatures began to warm and the pelagic ecosystem responded, with the biomass o f invertebrate zoop lankton increasing by a factor o f 2 from the early to late 1990s. This was followed by a sharp increase in the nekton biomass during the late 1990s, although this increase lagged that observed in the zooplankton. We conclude that the observed decline in cod recruitment since the late 1960s was due to a declining spawning-stock biomass caused in part by a deteriorating ocean environment. Furthermore, the subsequent increase in the abundance o f pelagic organisms observed during the latter half o f the 1990s is consistent with the expected biological response to changes in the physical ocean environment. Keywords: Atlantic cod, ocean environment, pelagic ecosystem, recruitment, survival, temperature. E. B. Colbourne and J. T. Anderson: Northwest Atlantic Fisheries Centre, PO Box 5667, S t John’s, Newfoundland, Canada A 1C 5X1 [e-mail:
[email protected]:
[email protected]]. Correspondence to E. B. Colbourne.
Introduction The latter half of the 20th century has been a time period of considerable variability in the marine ecosystem of the Newfoundland Shelf. In the years prior to the early 1970s the ocean environment was dominated by a general warming phase that reached its maximum by the mid-1960s. Beginning in the early 1970s, climate conditions in the Northwest Atlantic experienced near-decadal oscillations, with a general downward trend in ocean temperatures. In particular, the decade of the 1990s has experienced some of the most dramatic variations since meas urements began during the mid-1940s. During the same time period many commercial fish species also showed dramatic changes in abundance and distri bution, particularly during the decade of the 1990s. F or example, populations of Atlantic cod (Gadus
morhua) off Newfoundland decreased from all time highs in the 1960s to commercial collapse by the early 1990s. Many studies have suggested that variations in the physical ocean environment influence growth, recruitment, and distribution of many marine org anisms in Newfoundland waters (deYoung and Rose, 1993; Myers et al., 1993; Rose et al., 1994, 1995; Taggart et al., 1994; Narayanan et al., 1994; Colbourne et al., 1997; Carscadden et al., 2001). More recently, Parsons and Lear (2001) provided an overview of recent climate variability in the N orth Atlantic and its impact on the productivity of the marine ecosystem. However, overall, physical and biological interactions in the marine environ ment are usually non-linear and operate through complex mechanisms throughout the ecosystem over a broad range o f time scales. These interactions are
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further complicated by fishing mortality. Therefore, correlations between individual environmental indi ces with measures o f fish production often break down as different physical factors begin to influence various levels of the ecosystem and life stages of marine organisms (Mann and Drinkwater, 1994). However, long-term trends which coincide in the physical and biological environment may reflect significant change related to production in marine ecosystems and may provide some insight into physical-biological processes. Following the collapse of cod stocks off New foundland (NA FO 2J + 3KL) and on the Grand Banks (N A FO 3NO; Figure 1), fishing moratoria were put in place in 1992 and 1994, respectively. Fishing m oratoria provide a unique opportunity to observe and study the response of marine eco systems to variations in the physical environment in the absence of significant fishing mortality. In this article we first review the long-term trends in both the physical environment and cod abundance and survival during the 1960s to 1980s, noting that during this period it is difficult to disassociate envi ronmental influences from the effects of fishing mor tality. We then focus in more detail on the decade of the 1990s, examining variation in pelagic fish production in Newfoundland waters in relation to a warming physical environment.
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LO N G IT U D E
Figure 1. Regional map showing the positions of standard monitoring transects, Stn 27 and the statistical fish manage ment areas established by the Northwest Atlantic Fisheries Organization (NAFO).
Data The data utilized in this aticle were derived from three main sources: (1) spring and autum n bottom trawl surveys by the Canadian Departm ent of Fish eries and Oceans, (2) annual oceanographic moni toring surveys along standard sections, and (3) large-scale surveys of the pelagic environment on the Newfoundland Shelf. Canada has been con ducting stratified random ground fish trawl surveys on the Newfoundland Shelf in N A F O Divisions 2J3KLNO since the early 1970s (Doubleday, 1981; Bishop, 1994). The random stratified fish samples obtained from these surveys form a basis on which to determine recruitment and population abundance for demersal fish stock assessments. Oceanographic data were collected during these surveys at all fishing locations. Oceanographic measurements along standard sec tions on the Newfoundland and L abrador Shelves were initiated by the International Ice Patrol o f the US Coast G uard soon after the Titanic disaster in 1912 to monitor variations in the Labrador Current. Since the 1940s, oceanographic data have been collected on research surveys along standardized sections (Figure 1) under the auspices of the Inter national Commission for Northwest Atlantic Fish eries (ICNAF) by several countries and currently for the Northwest Atlantic Fisheries Organization (NAFO). Additionally, as part of an expanded Canadian Atlantic zonal oceanographic monitoring programme some of these transects are now sampled on a seasonal basis (Therriault et al., 1998). In recent years, 1994-1999, a comprehensive large-scale survey of the marine pelagic environment on the Newfoundland and Labrador Shelf has been conducted (Anderson and Dailey, 1997; Dailey and Anderson. 1997, 1998; Dailey et al., 1999, 2000). These surveys, initiated after the collapse o f the cod stocks o f Newfoundland, were designed to monitor the expected recovery o f Atlantic cod by providing a measure o f pre-recruit pelagic (0-group) cod as well as providing a full multispecies measure o f plankton and nekton for the study area. The surveys were carried out during late summer o f each year and also provided a comprehensive temperature and salinity survey of the shelf waters. Traditionally, two stocks of Atlantic cod have been managed off the east coasts of Newfoundland and Labrador. The northern cod stock (N A FO 2J + 3KL) was the largest cod stock in the Northwest Atlantic, ranging from southern Labrador to the northern G rand Bank. It was recognized that the northern cod stock was in fact a stock complex made up o f a number of population components. However, it was managed and assessed as a single
Biological response in a changing ocean environment in Newfoundland waters
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largely determines ocean climate variations through its influence on ice extent and duration, ocean tem peratures, and shelf stratification (Colbourne et al., 1994; Drinkwater, 1996). A convenient meteorologi cal index representing the strength of this circulation has been termed the North Atlantic Oscillation (NAO) index and is defined as the difference in the winter sea level air pressure between the quasistationary winter high and low pressure cells over the Azores and Iceland, respectively (Rogers, 1984). When the NAO index is strongly negative, warm saline ocean conditions generally prevail in the Northwest Atlantic and colder fresher conditions predominate in the Northeast Atlantic; and, con versely, when the NAO index is high positive. Spatial variations in the positions and extent of the pressure cells sometimes result in significant interannual variations in the strength o f the winter wind patterns in any one location. Overall, the NAO index accounts for only one-third of the total vari ance in the winter pressure anomaly field, but repre sents the most dominant signal of environmental variability for the N orth Atlantic (Dickson and Meincke, 1999). The NAO index is generally characterized by large amplitude fluctuations with periods ranging from annual to approximately 20-year cycles. Super imposed on these oscillations was a long-term dec line in the index from the early 1950s to the late 1960s, a period of generally warm saline ocean con ditions on the Newfoundland Shelf. This was also a time period of increased landings of Atlantic cod resulting from an expansion of the fishing industry (Figure 2A). Since the early 1970s, the NAO shifted towards more positive values and began to oscillate at quasi-decadal time scales superimposed on a gen erally increasing trend. As a result, the ocean envir onment in the Northwest Atlantic experienced more frequent periods of cold-fresh conditions following the 1960s. Landings of northern cod peaked in 1968, shortly after the NAO minimum, and subsequently declined to a minimum in 1977, increased during the 1980s and then collapsed to near zero by the middle 1990s (Figure 2A). The declines in the 1970s and 1990s have been attributed to high fishing mortality, first by foreign fishing prior to the extension of juris diction in 1977, and secondly to foreign and dom estic fishing in the 1980s and 1990s (Hutchings and Myers, 1994; Myers et al., 1996; Bundy, 2001). However, we note that the decline in cod landings also coincided with a changing ocean environment during this 20+ year period. The responses of the ocean on the Newfoundland Shelf to variations in the NAO are clearly evident Long-term trends by comparing long-term trends (Figures 2, 3). The The strength of the cyclonic atmospheric circulation combined effects of variations in air-sea heat fluxes, over the N orth Atlantic during the winter months storm forced mixing and intense winter convection
population using traditional analytical techniques. The southern Grand Bank cod stock (NA FO 3NO) is confined to the ice-free relatively warm waters of the southern G rand Banks. We considered these two cod stocks in relation to environmental variability because of the long time series of population esti mates based on Virtual Population Analysis (VPA) and the different ecosystems that they occupy. Recruitment (R) was defined as the abundance of cod at 3 years of age and spawning-stock biomass (SSB) was based on the total biomass of mature fish. D ata for recruitment and SSB were obtained from the most relevant assessment documents (Bishop et al., 1993; Stansbury et al., 1999). An index o f cod pre-recruit survival was estimated as the abundance of cod at age 3 divided by the SSB that produced each year class (R/SSB). We recognize the retrospective problems asso ciated with the VPA methodology, where the most recent year-class abundances are overestimated (Mohn. 1999). However, given the extreme declines in abundance of each stock in recent years, such esti mation errors are relatively small in our analysis. For the northern cod stock, the VPA models have not produced reliable estimates since 1993. There fore we have only presented data up to the 1990 year class. A research vessel trawl survey estimate is available for the offshore area since 1981, and it indicates that recruitment has remained at histori cally low levels since 1990 and population biomass has remained at extremely low levels since 1993 (Lilly et al., 1999). Collectively, these surveys provided a comprehen sive oceanographic data set tor the Newfoundland Shelf region with good temporal and spatial cover age for most of the decades since the 1950s. Oceano graphic data from other available sources archived at the Marine Environmental D ata Service (MEDS) in Ottawa were also used to define long-term means. In this article we present environmental time series as differences from their long-term averages (anom alies) referenced to a standardized base period from 1961-1990 (normal) in accordance with the conven tion of the World Meteorological Organization. In addition, we present correlations between environ mental indices and with indices of fish survival and recruitment (all of which show significant auto correlation) only to highlight the associations between long-term trends in fish production and the environment.
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E. B. Colbourne and J. T. Anderson -NAO ANOMALY
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