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ICES Marine Science Symposia, 215: 221-21)6 ICES Marine Science Symposia, 215: 221-21)6. 2002 Understanding the role of turbulence on fisheries production during the first century of ICES Brian R. MacKenzie MacKenzie, B. R. 2002. Understanding the role of turbulence on fisheries production during the first century of ICES. - ICES Marine Science Symposia, 215: 227-236. Since its inception, ICES has been concerned with the effect of hydrography on the abundance and distribution of fish and fish catches. One of the earliest and most sig­ nificant oceanographic findings made by the ICES community was the influence of vertical mixing and turbulence on seasonal plankton production processes. This dis­ covery, acquired over several decades of investigation, led to three major theories of fish population regulation and demonstrates the underlying impact that turbulence has on seasonal plankton and fish production. More recently, moderate levels of turbu­ lence and upwelling have been shown to produce the highest recruitment among clu- peid populations inhabiting major upwelling areas. The mechanism responsible for this pattern is a balance between the positive and negative effects of both turbulence and upwelling on plankton production, larval feeding, and advective processes. In one ICES upwelling zone (Bay of Biscay), recruitment of a local clupeid is related to some of these processes. This knowledge is contributing to the ICES assessment process for this stock. Frontal zones on continental shelves within the ICES Area are also moder­ ately turbulent environments, may also have an impact on fish recruitment, and have received particular attention by colleagues within the ICES community. In future, an understanding of how turbulence affects fish and plankton production at upwelling and frontal zones and during storms could help justify including additional environ­ mental and ecosystem information in recruitment and catch prediction models. Keywords: fish production, fronts. ICES, spring bloom, storms, turbulence, up­ welling. Brian R. MacKenzie: Department o f Marine Ecology and Aquaculture, Danish Insti­ tute for Fisheries Research, Kavalergården 6, DK-2920 Charlottenlund, Denmark; tel: +45 33 96 34 03; fax: +45 33 96 34 34; e-mail: [email protected]. Introduction developed during the first 100 years of ICES. To be con­ sistent with the theme of this session of the Symposium But the great fisheries of temperate seas probably ("Variability at all scales and its effect on the ecosys­ depend upon the start o f production in turbulent waters. tem"), I will consider processes covering both large and - D. H. Cushing (1989). small scales. Turbulence is an oceanic property which has a major influence on life in the sea. Phytoplankton species com­ position (Margalef, 1978), production rates (Lewis et Large-scale influences (decadal-century; a l, 1984), plankton foodweb structure (Kiørboe, 1993), whole basin) feeding rates (Dower et al., 1997; Kiørboe, 1997), and distributions (Lasker, 1975; Mackas et al., 1985; Seu- Arguably one of the most important influences of turbu­ ront et a l, 1999) have all been shown to co-vary with lence on fisheries production is its role in vertical mix­ turbulent intensity. Since turbulence is generated or sup­ ing and seasonal plankton production cycles in temper­ pressed by many different kinds of physical processes ate-boreal waters. In particular, turbulence generated by (e.g., tides, winds, buoyancy inputs, convection), the winds, tides, and convection is responsible for the re­ structure and functioning of aquatic ecosystems are plenishment of nutrients during the winter that allows commonly being perturbed by turbulent water motion of phytoplankton blooms to occur in the following spring various intensities and scales. (Cushing, 1975, 1989; Mann and Lazier, 1996). The ini­ In this paper, I describe how this turbulence affects tiation of these blooms depends partly on a relaxation of fish and fish production and how this understanding has turbulent mixing so that phytoplankton remain longer in 228 B. R. MacKenzie Production o f larvae Production of larval food Production o f eggs Time Figure 1. Schematic representation of Cushing’s match-mismatch hypothesis (see Cushing, 1990). The peaks illustrate the sea­ sonality of the abundance of fish eggs, larvae, and larval food. The striped area indicates temporal overlap between larvae and prey. the photic zone. The blooms lead to the seasonal in­ has been enormous. In fact, a century ago, many of the crease in zooplankton production and biomass which same workers who laid the organizational foundations becomes an important food source for fish larvae and for ICES were also instrumental in demonstrating that a juveniles, as well as for adult zooplanktivores (e.g., her­ seasonal plankton production pattern existed, that the ring, sprat; Cushing, 1975). seasonality itself depended on the availability, via verti­ The role of turbulence on the timing (i.e., initiation, cal mixing and turbulence, of nutrients for phytoplank­ delay, duration) of the spring plankton bloom in temper­ ton growth, and that the seasonality also depended on ate-boreal waters is central. Specifically, the reduction phytoplankton becoming exposed in the spring to prop­ in water column turbulence during the transition from er light and nutrient conditions. The fundamental obser­ winter to summer (e.g., due to reduced wind speeds and vations of phytoplankton abundance and distribution increasing temperatures) is critical for determining were made by scientists working in the late 1800s and when the onset of rapid phytoplankton growth occurs early 1900s (Mills, 1989). These scientists included after nutrients have been re-mineralized and re-supplied Hensen, Brandt, and Lohman in Kiel, Gran and Braarud to the surface layer during the autumn-winter mixing in Oslo and St. Andrews (Canada), Whipple in Boston, period (Mann and Lazier, 1996; Huisman et al., 1999). Riley in Woods Hole, and Sverdrup in Norway. Excessive turbulence in the spring will delay the initia­ One of the key roles that ICES played after its incep­ tion of the phytoplankton bloom because phytoplankton tion was the development of standardized techniques for cells will be mixed below the photic zone where light- measuring nutrient concentrations (Mills, 1989). These limitation will suppress growth rates. As a result, a techniques were spread among colleagues by meetings sequence of turbulence "events" (i.e., strong followed by and courses arranged by ICES and, for the first time, calm) is necessary to initiate the spring plankton bloom. enabled comparisons of phytoplankton abundance/dis­ This sequence of events (re-mineralization and strong tribution with nutrient concentrations (Mills, 1989). winter mixing that replenishes nutrients followed by Based on the empirical field observations, it was possi­ greatly reduced spring mixing that allows phytoplank­ ble to demonstrate that phytoplankton abundance was ton to remain in the photic zone) is a highly regular and associated with some of the variations in nutrient con­ predictable feature of temperate-boreal habitats. As a centrations. These observations were key to demonstrat­ result, these environments are characterized by strong ing that the spring bloom was related to vertical mixing seasonal pulses in plankton production, although their and stabilization processes (e.g., Gran and Braarud, exact timing, magnitude, and duration can vary greatly 1935; Riley, 1942; Sverdrup, 1953). from year to year. Nevertheless, at decadal-century time Notably one of the motivations to develop an interna­ scales and over large areas of the sea, they are consistent tional body for marine science investigations was the features that provide important environmental signals to desire to improve the possibility for predicting fluctua­ other biota in the ecosystem. tions in the important fish stocks of northern European The role of ICES in defining these links between tur­ waters. It was understood at the time that hydrographic bulence, plankton production, and fisheries production variability was part of the cause of such variations (e.g., Understanding the role of turbulence on fisheries production during the first century of ICES 229 1985/1986 1986/1987 1987/1988 90 60 60 40 0)Ï a)» £= F 30 20 Ü E 0 0 40 42 44 46 48 50 52 54 56 42 44 46 48 50 52 54 56 42 44 46 48 50 52 54 56 ™ 2.4 a. O) »> I 0.6 o 32 34 36 38 40 42 44 46 48 34 36 38 40 42 44 46 48 34 36 38 40 42 44 46 48 Figure 2. Total numbers of a newly settled temperate marine reef fish (Heteroclinus sp.) in Storm Bay, Australia, during three reproductive seasons and the abundance of chlorophyll a. Note that the scales on the ordinate axes are offset by 8 weeks corre­ sponding to the duration of the larval period before metamorphosis and settlement. Line breaks represent missing data. See Thresher et al. (1989) for details. Hjort, 1914), but the mechanisms were only vaguely strate that fish recruitment was strongly correlated known and not quantified (e.g., owing to changes in with phytoplankton production peaks following storm- timing and location of migration routes, effects on egg induced mixing (Figure 2). Other colleagues have and larval mortality rates). However, as more knowl­ observed relationships between larval/0-group growth edge of the spring plankton bloom became available, and recruitment (Campana, 1996; Ottersen and Loeng, attention began to focus on the role of this event in fish 2000), but it is unclear to what extent the growth rates growth and survival. were driven by variations in temperature or food supply. Cushing (1975, 1990) observed that many of the most Nevertheless, the influence of turbulence on seasonal abundant fish species in temperate-boreal waters (e.g., plankton production and fish ecology is great. It seems cod, haddock, herring) spawned in the spring during or obvious that large parts of continental shelf regions shortly after the spring plankton production peak and would be considerably less productive if turbulence dur­ that spawning times were relatively stable compared ing winter were too weak to remix the water column and with the temporal variability in onset of the spring resupply the photic zone with nutrients.
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