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Fishery Production and the Mississippi River Discharge

Churchill B. Grimes

Most fishery production worldwide is confined to coastal regions and is associated with three nutrient enrichment processes: 1) coastal upwelling, 2) tidal mixing, and 3) land-based runoff and major river outflow (Caddy and Bakun, 1994). Evidently, terrestrially enriched river discharge favorably influences the biological processes (i.e., growth, mortality, and recruitment) that underlie fishery production, and the physical oceanography associated with the discharge of fresh water into coastal seas also can significantly influence fishery production (Govoni et al., 1989; Govoni and Grimes, 1992; Grimes and Finucane, 1991; Grimes and Kingsford, 1996). Fisheries associated with the continental shelf waters surrounding the Mississippi River outfall are a good example of the result of these biological and physical processes. Landings from the rich grounds surrounding the river delta, referred to as the fertile fishery crescent by the late Gordon Gunter, account for 70-80% of the 1.8 billion-pound annual commercial catch from the Gulf of Mexico, second only to Alaska in the United States (National Marine Fisheries Service, 1998).

Figure 1. Salinity profile along a transect of the Mississippi River plume from inshore (left) to offshore (right).

Strong circumstantial evidence of the relationship between river discharge and fishery production is that when flows have been controlled or eliminated, major declines in fisheries have followed. For example, filling of the Aswân High Dam on the Nile River began in 1965 and was completed in 1969, during which time flow decreased by 40 km³ yr^-1, and there was a concomitant decline in primary production off the Nile Delta. Egyptian fishery landings in the Mediterranean Sea declined from 37,800 metric tons in 1962 to 7,142 metric tons in 1976, with an attendant simplification in community structure (Bebars and Lasserre, 1983). Declines in fisheries associated with the Nile River included valuable prawn fisheries (Bishara, 1984) and the sciaenid fish Argyrosomus regius whose major reproduction and nursery areas were off the Nile Delta (Quero, 1989). Similarly, the totoaba, Totoaba macdonaldi, the largest sciaenid in the world (2 m and 100 kg), once supported important commercial and recreational fisheries in the Gulf of California. In 1976 the totoaba was placed on the endangered species list, its decline attributed primarily to the diversion of the Colorado River which drastically altered spawning and nursery areas near the delta (Barra-Guevara, 1990). There is also evidence that fishery declines can be reversed when river flow is restored. For example, in India the Farakka barrage was constructed on the Ganga River in 1975 to increase freshwater discharge into the Hooghly distributary, resulting in a twofold increase in overall annual fishery landings from the estuary (Sinha et al., 1996). Similarly, after filling of the Aswân High Dam and releases of water from the dam increased river discharge, landings recovered to about one-third of the pre-dam level (Smetacek, 1986); by 1989 landings had recovered fully to preconstruction levels, but under a different fish community structure (Lasserre et al., 1997).

Figure 2. Discharge of the Mississippi River into the Gulf of Mexico. The shallow turbid plume is disturbed by the action of the propellor of a tug boat crossing the turbidity front exposing underlying shelf water.

The fish community of the northern Gulf of Mexico can be divided into four principal ecological groupings: estuarine-dependent, coastal, reef, and oceanic species groups (Dagg et al., 1991). Estuarine-dependent species such as Gulf menhaden, southern flounder, red drum, spot, Atlantic croaker, spotted sea trout, and striped mullet typically spawn in neritic waters followed by transport of larvae and young juveniles to estuaries. The coastal group is comprised of a large number of ecologically and economically important species, including king and Spanish mackerels, bluefish, coastal herrings, little tunny, crevalle jack, blue runner, butterfish and hakes, that complete their life cycles in the pelagic and benthic realm associated with the continental shelf. Populations of the latter two groups are those most associated with the river discharge, and because their life cycles are wholly or mostly carried out in waters directly influenced by the river discharge, critical demographic processes regulating population size such as growth, mortality, and recruitment are potentially affected. For example, recruitment of Gulf menhaden, a well-studied species of the estuarine-dependent group, was shown to be elevated after 1975 when river discharges increased and became more variable, perhaps due to enhanced primary and secondary production driven by increased nutrient influx from the Mississippi River (Govoni, 1997). Nourished by the Mississippi River sediment loads, vast coastal wetlands have formed near the past and present deltas (Reed et al., 1995). These wetlands provide nursery habitat for juveniles of many estuarine-dependent species such as menhaden, sea trouts, blue crabs, penaeid shrimps, and others (Zimmerman and Minello, 1984; Baltz et al., 1993, 1998), and no doubt play an important role in enhancing the production of these species (Chesney et al. , in press). Coastal species, like king mackerel, complete their life cycles in Gulf of Mexico shelf waters that are influenced by the discharges, and consequently their production may be influenced by the river discharge. Because king mackerel larvae are most abundant (Grimes et al., 1990) and exhibit faster growth (DeVries et al., 1990) in Gulf of Mexico shelf waters around the Mississippi River delta, their recruitment has been tentatively linked to the Mississippi River discharge (Finucane et al. 1990; DeVries et al., 1990; Lyczkowski-Shultz and Link, unpublished manuscript).

Figure 3. Comparison of average copepod nauplius densities in and away from the Mississippi River plume in the Gulf of Mexico (from Dagg et al. , 1987, and Dagg and Whitledge, 1991).

The Mississippi-Atchafalaya River system influences a large portion of the continental shelf in the northern Gulf of Mexico. The largest river system in North America, its drainage basin includes 43% of the contiguous United States and parts of two Canadian provinces. It annually discharges an average 1.83 x 10⁴ km³ (Gunter, 1979) of fresh water, nutrients and suspended materials forming a shallow plume that is detectable up to 100 km offshore (Riley, 1937). Discharge varies seasonally, being largest in spring, decreasing through summer and fall to a winter low (Dinnell and Wiseman, 1986). All of the Atchafalaya distributary and 30% of the Mississippi discharge flows westward along the Louisiana-Texas shelf (Dinnell and Wiseman, 1986). During most of the year, the westward flow hugs the coastline and flows into Texas and even Mexico. During summer, strong southerly winds along south Texas tend to push water back towards the Louisiana shelf, reversing the westward flow (Wiseman et al., 1997).

The near field environment of the discharge plume has a characteristic hydrographic structure created by the abutment of water masses of very different densities. Lighter plume waters are represented by a shallow lens of low-salinity water overlying heavier high-salinity Gulf of Mexico shelf water (Grimes and Finucane, 1991; Govoni and Grimes, 1992). Isohalines are closely spaced and approach the surface in the six to eight kilometer wide frontal or mixing zone between the shelf and plume waters (Figure 1).

Turbidity fronts (5 to 100 meter scale), represented by sharp color discontinuities, are the seaward projection of concentrated suspended particulate matter, and they are often nested within the frontal zone (Figure 2). These features are ephemeral, forming and dispersing along tidal frequencies, and are in contrast to the more or less permanent frontal zone (Govoni and Grimes, 1992).

Figure 4. The distribution of phytoplankton biomass with surface salinity in the Mississippi River discharge plume (redrawn from Dortch and Whitledge, 1992).

Figure 5. The distribution of copepod nauplius density with surface salinity in the Mississippi River discharge plume (redrawn from Dagg and Whitledge, 1991).

Primary and secondary production and fish larvae are concentrated in the vicinity of the plume in general, and in the frontal region in particular. For example, average copepod nauplii densities off the Mississippi River and its plume can be up to one to two orders of magnitude higher than in the Gulf of Mexico east or west of the river delta (Figure 3). Highest values for phytoplankton biomass (chlorophyll) (Lohrenz et al., 1990; Dortch and Whitledge, 1992), primary productivity (Lohrenz et al., 1990), and copepod nauplii concentration (Dagg and Whitledge, 1991) all occurred at intermediate salinities, i.e., in the frontal zone (e.g., Figures 4 and 5).

Surface waters converge at the plume front, primarily due to strong horizontal density gradients (and consequently pressure gradients) that are produced within and below the frontal layer. Cross frontal circulation is characterized by vigorous convergence on both sides of the front, typically higher on the high density (shelf water) side than the low density (plume water) side, e.g., 0.2 and 0.1 m sec^-1, respectively (Govoni and Grimes, 1992). As surface waters converge, planktonic organisms move passively with the water toward the front where converging water masses move downward with gravity. Surface-seeking and buoyant organisms accumulate at the surface as they resist downward movement. This is a local, but important, transport mechanism that can concentrate larval fish and zooplankton and account for the high densities of these properties observed in the frontal zone (Figure 6). Govoni and Grimes (1992) measured surface convergence velocity at turbidity fronts of up to 0.8 m sec^-1. Observed velocity was always greater than velocity calculated from the density gradient alone (Figure 7), because observed velocity is the sum of density-driven velocity plus the tidally-driven velocity inherent in shelf waters. They used the advection diffusion model of Olson and Backus (1985) to simulate surface densities of fish larvae at the front that agreed well with observed values.

Although it seems clear that the nutrient-rich discharge of the Mississippi River enhances fishery production, the exact mechanism is less clear. The traditional way of formulating fishery production states that production equals growth and recruitment minus fishing and natural mortality. It is generally accepted that the level of recruitment is established during the first few months of life, and, of the above four factors controlling production, recruitment is the natural process that makes the largest contribution to changes in stock production.

Figure 6. Densities of fish larvae in different seasons in the vicinity of the Mississippi River plume (from Ditty, 1986), in plume, front and shelf waters (from Grimes and Finucane, 1991), in frontal water (from Govoni et al., 1989), and away from the discharge influence (Loop Current) in the open Gulf of Mexico (from Richards et al.,1989).

Thus, it is most likely that the Mississippi River discharge enhances fishery production principally through its influence on recruitment. Having observed the previously described ecological conditions in the plume, Grimes and Finucane (1991) developed a modification of the short food chain hypothesis (Legendre and LeFevre, 1989), explaining how the Mississippi River might act to enhance recruitment of associated fish larvae. This hypothesis states that fish larvae concentrated in the vicinity of the plume in general, and in the frontal region in particular, take advantage of abundant food resources and consume a superior diet, grow faster and thus experience a shorter larval stage duration and survive better.

Implicit in this hypothesis is that fish larvae in the vicinity of the discharge plume are not advected away from the rich plume environment by the average westward flowing surface currents that prevail off the Mississippi River delta (Dinnell and Wiseman, 1986). Hitchcock et al. (1997) deployed radio-tracked surface drifters in the vicinity of the plume that became entrained in a tongue of Gulf of Mexico shelf water that intruded into the Louisiana Bight and rotated clockwise. Nitrate, phosphate and silicate distributions in shelf and plume waters verified the origin and rotation of the shelf water. At least in this instance, the clockwise circulation that existed in the vicinity of the plume would have acted to retain fish larvae.

Figure 7. Hydrodynamic convergence at turbidity fronts of the Mississippi River discharge plume. Observed values were measured using surface drifters and potential values calculated from the density gradient.

Research findings are not totally in accord with the first element of the short food chain hypothesis, i.e., it cannot be stated unequivocally that fish larvae associated with the Mississippi River plume are conferred a trophic advantage. Spot, Leiostomus xanthurus, larvae collected off the Mississippi River plume ate twice as many food organisms as did larvae in Gulf of Mexico shelf waters (Govoni and Chester, 1990). However, organisms within the plume were mostly small (tintinnids, copepod nauplii, pelecypod veligers and invertebrate eggs), whereas organisms eaten in shelf waters were larger (copepodites and adult copepods). Because the volume and nutritional quality of gut contents of larvae from the two areas were roughly equivalent, they concluded that larvae in the plume gained no trophic advantage. Powell et al. (1990) used morphological, gut content, and recent growth criteria to evaluate nutritional condition of spot larvae associated with the Mississippi discharge, and could not consistently demonstrate an advantage. Conversely, a diet study on striped anchovy, Anchoa hepsetus, collected along transects crossing plume, front, and shelf waters showed that diatoms and copepods were the dominant food items (accounted for 60.4, 69.5 and 81.2% of total prey items in plume, front and shelf samples), and that the larger, more nutritious copepods occurred more frequently and accounted for the highest percentage of food items in guts of larvae collected in frontal waters, followed by plume waters and then shelf waters (Figure 8) (McNeil and Grimes, 1995).

Figure 8. Frequency of occurrence (%) in diet and number (%) of the principal food categories (diatoms and copepods) in the diet of striped anchovy, Anchoa hepsetus.

A suite of biochemical indices to nutritional condition (RNA/DNA ratio, percent protein and CS and LDH enzyme systems) were examined on striped anchovy collected along the same transects off the Mississippi plume; larvae collected in frontal waters were in the best nutritional condition (Geiger, 1999). Furthermore, in a recent review of the influence of riverine plumes on fish larvae worldwide, Grimes and Kingsford (1996) found that certain fish taxa, e.g., small opportunistic species, appear to be associated with plumes and may be better adapted than larger, more competent larvae of other species to take advantage of abundant food resources around plumes and their fronts.

The second element of the hypothesis states that fish larvae that are conferred a trophic advantage will respond by growing faster, and there is some evidence that growth of some fish larvae, as determined from otolith microstructure, may be enhanced. Growth of king mackerel, Scomberomorus cavalla, was higher off the Mississippi River plume (0.95 mm d^-1) than at other locations in the Gulf of Mexico (0.79 mm d^-1) (DeVries et al., 1990). However, superior growth off the plume was not demonstrated for Spanish mackerel, S. maculatus (DeVries et al., 1990) or little tunny, Euthynnus alletteratus (Allman and Grimes, 1998). Other results on Spanish mackerel (Grimes and DeVries, Figure 9), as well as those on yellowfin tuna, Thynnus albacares (Lang et al., 1994), and striped anchovy, Anchoa hepsetus (Day, 1993), suggest that larvae associated with the Mississippi plume grow faster at intermediate salinities, i.e., frontal waters (0.6 vs. 0.75 and 0.5 vs. 0.67 mm d^-1).

Figure 9. Mean growth per day of Spanish mackerel, Scomberomrus maculatus, larvae collected in the vicinity of the Mississippi River discharge plume plotted on salinity.

The final element of the hypothesis states that faster growth leads to shorter duration of the larval stage and better survival, with the caveat that the same dynamics that concentrate prey of fish larvae might also concentrate their predators. There is little evidence to evaluate this element of the hypothesis. Grimes and DeVries (unpublished data) estimate instantaneous rates of natural mortality for Spanish mackerel and king mackerel using a catch-curve approach (i.e., regressing the log of frequency on age of the descending limb of age-frequency histograms). Instantaneous natural mortality estimates were approximately 0.3 d^-1 away from the plume and 0.6 d^-1 or higher in the vicinity of the plume. For little tunny, instantaneous natural mortality was higher in the vicinity of the Mississippi River plume (0.95 d^-1) than in the Gulf of Mexico off Panama City, Florida (0.72 d^-1) (Allman and Grimes, 1998). Similar analyses for striped anchovy in water masses off the Mississippi River suggested that natural mortality in the front (0.13 d^-1) and plume (0.23 d^-1) may be higher than that experienced in shelf waters (0.09 d^-1) (Day, 1993). Conversely, yellowfin tuna (Lang et al., 1994) experienced higher natural mortality at fronts (0.41 d^-1) than in the plume (0.16 d^-1). These differences in mortality rates should be interpreted with caution because application of catch-curve or survivorship analysis to estimate instantaneous mortality rates assumes equal vulnerability to capture by the sampling gear for all ages used in the analysis. Faster growth rates might lead to biased mortality estimates because fast-growing, larger larvae become less vulnerable to capture and may be underrepresented at the older ages used in the analysis. Although these results are tentative, they do not support the contention that higher growth rates of larvae associated with river plumes lead to better survival.

To summarize the results of evaluating the elements of the short food chain hypothesis with respect to the Mississippi River, it appears that some species of fish larvae, opportunistic ones, are able to take advantage of abundant prey resources. Also, some species of fish larvae appear to grow faster, but mortality rates may also be higher. So, whether this hypothesis is valid and the population dynamics of fish larvae in the vicinity of river plume favor recruitment, depends upon the relative magnitude of growth and mortality. Incremental changes in mortality will have a much larger effect on survival and recruitment than incremental changes in growth (see Grimes and Kingsford, 1996). So, if physical and biological conditions in the vicinity of the Mississippi River plumes aggregate larval fish prey that results in a trophic advantage and faster growth, but also aggregates predators and increases the mortality rate on larvae, the disadvantage of increased mortality may well outweigh the advantage of faster growth, and increased survival and recruitment will not be the result.

In summarizing their review of the effects of riverine plumes on fish larvae and their recruitment dynamics, Grimes and Kingsford (1996) offer two alternatives to the short food chain hypothesis. One alternative to explain the apparent favorable effect of river plumes on fish production, the total larval production hypothesis, is that trophic conditions support such high total production of fish larvae that negative effects of unfavorable dynamics are overridden. That is, high primary and secondary production associated with plumes may simply support such high total production of fish larvae that the specific population dynamics at plumes are not often relevant. A second alternative is that plumes and associated circulations facilitate the retention of larvae within an area. The presence of food would of course be important, but variation in physical retention rather than production may explain variation in recruitment, as argued for the member/vagrant hypothesis of Sinclair (1988).

In conclusion, there is strong circumstantial evidence worldwide that nutrient-enriched riverine discharges enhance fishery production on the adjacent continental shelves. This is also apparently the case with the Mississippi River where 70-80% of Gulf of Mexico fishery landings come from waters surrounding the Mississippi delta. Two major fish species groupings, estuarine-dependent species and coastal species, are those most likely to be influenced by riverine discharge. While riverine enhancement of fishery production seems clear, the exact mechanisms through which it occurs are not. I argue that since recruitment is the natural process that makes the greatest contribution to fish stock biomass, it is by improving recruitment that fishery production is probably enhanced. Recruitment could be increased if fish larvae in the plume environment experience enhanced feeding conditions, grow faster, and thus experience a shorter larval stage and survive better. Waters influenced by the river discharge are a rich environment where both physical (hydrodynamic convergence) and biological dynamics (primary and secondary production) may favor the processes that regulate recruitment, i.e., larval feeding, growth, and survival. Feeding and growth of some species of fish larvae are enhanced, but in other species they are not, and larval survival may be lower in the vicinity of the plume, perhaps because the advantage of higher growth from concentrated prey is outweighed by increased predation from concentrated predators.

References

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About the Author

Dr. Churchill B. Grimes has been Director of NOAA fisheries newest laboratory, the Southwest Fisheries Science Center Santa Cruz/Tiburon Laboratory, since June 1998. Prior to his current position, he was on the staff of the National Marine Fisheries Service, Southeast Fisheries Science Center, Panama City (Florida) Laboratory for 15 years where he served as Director from 1993-1998 and was head of Fishery Ecology from 1985-1993. Before becoming employed by NOAA, Dr. Grimes was an Associate Professor of Fisheries Science at Rutgers University in New Brunswick, New Jersey. A native North Carolinean, Dr. Grimes received his B.S. and M.S. degree in Biology from East Carolina University in Greenville, North Carolina, and his Ph.D. in Marine Science from the University of North Carolina in Chapel Hill. His career in teaching, research and administration has spanned over 30 years. He has published over 70 articles on diverse research topics, including life history, population dynamics, habitat ecology, behavior, recruitment dynamics and fishery oceanography of deep-water reef fishes, coastal pelagic fishes, as well as coastal and estuarine dependent species. He has been an active member of the principal professional fisheries organization, the American Fisheries Society, for nearly 30 years, having served as the President of the Marine Fisheries Section and been awarded the Southern Division Outstanding Achievement Award. Dr. Grimes has continually been active in graduate education having served in adjunct faculty status at the University of Delaware, Florida State University, the University of Florida, Louisiana State University, the University of South Florida and the University of California at Santa Cruz.

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