Satellite observations have relevance for fisheries not by direct fish location, but through the characterization of environmental factors that affect fish habitats. Environmental parameters that are well measured by data from recent and current orbital instruments include surface temperature (NOAA-AVHRR), ocean color (Nimbus 7-CZCS, SeaStar-SeaWiFS), and wind and current data (TOPEX/Poseidon, Seasat). Key advantages of satellite-based observation include: 1) Large areas can be imaged at once -- discrete observations can cover a range of scales; 2) Observations can be repeated frequently; and 3) Observations can be made independently of the weather.
Remote sensing applied to fisheries requires previous knowledge of habitat preferences of the fish, biological quality of the waters, oceanography of the area (the strength, orientation, evolution and duration of ocean features such as fronts, eddies, etc.), behavior of a given species at various temperatures, and catch rates occurring under those conditions. The effects of the environment on any given species are location- and season-specific. Long time series of oceanographic and fish population data are then very important in determining the success of satellite oceanography applied to fishery management.
The most effective way to illustrate the full spectrum of industry-specific applications that MTPE/EOS instruments will empower is through use of an applications matrix (Figure 1). In this table, the rows correspond to discrete, independent applications and the columns correspond to individual MTPE/EOS instruments. The existence of a "hit" between an application and a specific instrument is denoted by an oval symbol. The variation in the robustness of the applications is not indicated; the applications thus indicated span the complete spectrum from experimental applications of significant promise to applications of proven maturity.
MTPE/EOS industry applications scenario descriptions are directly tied to this matrix. Individual instrument-crossing applications consist of a traverse along a row. The application scenarios described below are tied by numbers to the corresponding matrix; they are grouped where possible. They focus on knowledge outputs from the MTPE/EOS instruments and the specific application outcomes.
Experience with CZCS demonstrated the utility of ocean color images to distinguish distinctive water masses, currents, and circulation patterns. Subtle changes in the "blueness" and "greenness" of water can be dramatically enhanced by instruments specifically "tuned" to the task, such as SeaWiFS (and to a lesser degree MODIS). As natural waters become richer in biomass, they become "greener," as they lose biomass, they become "bluer." Currents and circulation patterns can be understood through analysis of diagnostic features on these special "still photos" and through analysis of sequences of images.
Indirect observations of currents and circulation patterns will be made by measuring oceanic wind fields with SeaWinds because sea surface winds and currents are often coupled, and measuring ocean height with EOS ALT because currents cause subtle but characteristic changes in sea surface topography. In addition, EOS models will aid in the prediction and explanation of currents through assimilation of satellite data with numerical simulation of weather.
Upwelling water masses indicated by ocean color fronts attract aggregations of fish populations (e.g., albacore tuna populations in the Pacific Ocean). There are no recent ocean color data that are readily available (CZCS operated from 1979 to 1986). Near-term improvements in quality and availability of ocean color data will be realized with the launch of SeaWiFS in late 1996.
Early detection of oceanic features can avoid gear loss (e.g., the primary causes of unusual gear loss suffered by lobster and crab fishermen appear to have been ocean currents related to large scale features, such as eddies with diameters between 70 and 270 km, with rotation flows than can reach speeds of over 2 knots).
Maps of Sea Surface Temperature (SST) are directly relevant to fisheries because of the temperature preferences of individual fish species, and the fact that maps of SST reflect currents and zones of upwelling or downwelling water. This information is so important that operational ocean surface temperature maps derived from satellites are routinely used by sports fishermen to plan outings. On an industrial scale, these data affect strategic planning.
Because temperature and currents are related, sea surface temperature maps help refine understandings of currents and circulation described above (e.g., the identification of ocean circulation mesoscale features such as cold/warm-core rings; in a warm core ring, warm water species of fish such as shark and swordfish congregate there; they find more food along the rotating perimeter of warm eddy). These data also indirectly aid fishermen and fisheries managers because sea surface temperature is directly related to weather (warm water aids storm formation) and climate (El Niño is arguably the most climatically important oceanic thermal anomaly on Earth).
Sea surface temperature can be obtained directly from satellite remote sensing. The long wave (thermal infrared) radiation emitted by an object is mathematically related to the temperature of that object. The MODIS instrument includes several bands sensitive to the thermal infrared, allowing precise, daily measurements of SST over the world's oceans.
Knowledge of surface winds benefits fishermen by aiding ship navigation, voyage planning, and anticipation of sea state. SeaWinds is designed to provide direct measurement of surface wind fields over water by pulsing radar signals and measuring the backscattered return. The strength of the return signal is primarily controlled by surface roughness of the water, and hence, local wind speed. In addition, mesoscale atmospheric models constrained by EOS data will offer more accurate local predictions of surface wind fields.
Water containing suspended sediment exhibits a characteristic increase in the visible and near-infrared (VNIR) reflectance. The high spectral resolution of MODIS and SeaWiFS in the VNIR spectral region makes these instruments ideally suited to gather these data and detect subtle changes in sediment concentration that are undetectable by the human eye.
On a local scale, near-shore marine bathymetry in relatively clear water can be seen directly by visible and near-infrared instruments. Maps derived from these measurements made over the last 20 years have been used for many specific fisheries applications.
Life on Earth started in the oceans and the oceans continue to host the base of the Earth's food chain. Phytoplankton thrive on organic carbon in seawater and utilize chlorophyll to power their digestion of carbon. MODIS and SeaWiFS have individual bands tuned to the reflectance patterns of chlorophyll and chlorophyll fluorescence, allowing improved species identification and biomass estimates.
Phytoplankton thrive in nutrient-rich upwelling regions that are well identified by their color, temperature, and current signature. Cold, nutrient-rich water results in high food organism densities. This attracts an aggregation of fish populations (e.g., the aggregation of salmon off the southwest coast of Vancouver Island is tied to prevailing circulation patterns and areas of upwelling).
Short-term fish habitat characterization is based on a combined understanding of currents, circulation patterns, winds, sediment concentrations, water temperature, and nutrients. As described above, SeaWiFS and MODIS will routinely provide direct information on all of these parameters. Long-term monitoring (of habitat health and variability) is one of the principal program drivers.
Coral reefs are to the ocean what tropical rain forests are to the land. They host some of the greatest biodiversity within the ocean. They provide rich habitats for large numbers of fish species that are either commercially important or critical components of the fish food chain. Intelligent management requires accurate mapping of these shallow water features. Because coral reefs only form in relatively shallow and clear water, they are well imaged in the visible and near infrared. Repeat mapping will aid the early identification of physical changes to the reefs that could occur in response to either natural or human factors. In addition, sediment loading and pollution represent major threats to coral reefs, and these factors can be monitored directly from space.
Pollution of oceans, rivers, and lakes is one of the principal threats to fisheries worldwide. Management of fish habitats to optimize food yield is a global goal that benefits society. Point-source pollution can often be directly "seen" on visible and near-infrared satellite images (e.g., from MODIS and SeaWiFS without substantial image processing. Hydrocarbons (e.g., oil) are particularly well detected by remote sensing. The contrast in emissivity between an oil slick and the surrounding water causes distinct anomalies in the thermal-infrared part of the spectrum, which can be readily detected with MODIS.
Improved weather and storm prediction provide direct benefits to fishermen, allowing improved navigation and logistical planning. This aids in cost minimization by enhancing storm preparation and minimizing storm damage. EOS instruments can help provide improved weather prediction both directly (e.g., tracking of wind fields via SeaWinds) and indirectly (e.g., improved performance of numerical weather models constrained by EOS data).
