By Dr. Milton Love
University of California, Santa Barbara
Fishes are extremely sensitive to their environment. If you think about it, a fish's life is divided into only three parts: 1) Eating, 2) Avoiding being eaten, and 3) Reproducing. Everything else is just window dressing for these three activities. What I will do is go through various environmental factors in a fish's life and explain how it might react, and how this impacts a fisherman.
The major environmental factors in a fish's life are: 1) water temperature, 2) water clarity, 3) water motion, 4) water salinity and 5) light levels (both daily and seasonally). These water parameters are caused by six phenomena: 1)currents, 2) waves and swells, 3) time of day, 4) time of year, 5) tides and 6) rainfall. Obviously, some of these phenomena produce more than one effect. For instance, when an El Niño occurs, water temperature rises, but so does water clarity. During storms, waves cause more water motion near shore, which causes sand and mud to be kicked up, resulting in a decline in water clarity. Time of year influences rainfall, light levels, water motion, water clarity, water temperature etc. A full moon produces more light at night, but it also produces larger tides.
Currents are a major factor in a fish's life for a number of reasons. Off California, there are two major currents. First, there is the California Current, a cold current which sweeps down the coast as far south as Pt. Conception, then swings offshore. During winter and spring, the California Current is at its strongest and parts of it enter southern California waters. There is also the Davidson Current, a flow of warmer water which moves northward from Baja California and primarily bathes southern California and parts of central California during summer and fall. Periodically, there is an El Niño, which is a mass of warm water that moves northward from a region near the equator.
How and why do these factors influence fish behavior? Let's take them one at a time.
Water temperature - In any part of the ocean, water temperature is controlled by three major factors: 1) water depth, 2) energy from the sun, and 3) currents. In the case of water depth, the deeper the water, generally the colder it is. Energy from the sun warms the water; during summer there is more energy available than during the winter. Currents also tend to be seasonal in nature. The cold California Current is strongest in winter and spring, and the warmer Davidson Current is strongest in summer and fall.
In addition, the ocean waters near the coast of California are sometimes subject to upwelling, a special kind of current. In upwelling, winds help blow surface ocean water away from the coast and cold deep water replaces it. This occurs primarily during winter and spring and may cause a very rapid decrease in temperature. Tides also influence water temperature, because they influence current speed and direction.
For instance, as the tide goes out, warm inshore water may be transported offshore. By the same token, incoming tides may bring in cold water. Surprisingly, tides can influence water miles offshore and many hundreds of feet down.
Most fishes are cold-blooded. That is, their body temperature is the same as their environment. This means that the chemical processes in their bodies are greatly influenced by water temperature. When the water is warm, their internal processes tend to speed up. This means they tend to be more active and require more food. On the other hand, when temperatures decrease fishes often slow down and become torpid, they require less energy and they feed less. Fishes often seek out the temperatures they prefer.
Changes in water temperature are also often cues for reproduction. Many fishes start to develop eggs and sperm during either spring or fall, when there tend to be either rapid increases or decreases in water temperature. In turn, fishes often change their behavior when they are breeding. For instance, they may migrate to spawning grounds, change their position in the water column or form large schools.
Fishes may also be attracted to particular water temperatures because food may be more available there. For instance, areas where two water masses meet (these are called oceanic fronts) are characterized by places where surface water temperature changes very rapidly. During the summer and fall off California, a typical oceanic front may exist perhaps 100 miles from the coast, where the California Current and Davidson Current brush against each other. These areas tend to have large amounts of plankton, which in turn attract small fishes, such as anchovies and sardines, and these attract large fishes, such as tunas and swordfishes.
How might a fisherman use a knowledge of water temperature to increase catches? Many fishermen, particularly those chasing pelagic fishes such as tunas and swordfish, now use satellite images of the California coast which show sea surface temperatures. They look for regions where warm and cold oceanic fronts meet and they fish there. Mako shark and swordfish fishermen know that these species tend to stay on the warmer side of the temperature break, while blue sharks often remain on the cooler edge. This knowledge helps them target makos or swordfish but avoid blues, which are largely unmarketed.
During the 1983 El Nino, California halibut moved northward from southern California into central California following the warm current. Catches of California halibut in central California were higher than they had ever been before, while those in southern California decreased dramatically. In a future El Niño, halibut fishermen who recognized this movement might increase their effort off central California.
Fishermen from the beginnings of time have recognized spawning aggregations and many have used an increase (or decrease) in water temperature to begin searching for these schools. Herring fishermen in San Francisco Bay harvest fish which come into the Bay to spawn, and their spawning is partially a response to changes in water temperature. Hook and line fishermen, who depend on fishes being hungry, are particularly sensitive to temperature fluctuations. Fishes such as barracuda, will virtually stop feeding when temperatures suddenly drop. In the past, large numbers of barracuda were taken by trolling lures and troll fishermen were acutely aware of temperature changes. Often they did not even try trolling when cold water set in. On the other hand, as temperatures rose, their effort increased.
Fish behavior may also vary with temperature. Swordfish harpooners capture swordfish as they lay on the surface. However swordfish may not come to the surface if water temperatures stay high. Rather, they will stay underwater, where temperatures are cooler. Harpooners may look for cooler water, avoiding too warm conditions.
Water clarity - Water clarity, which is basically how far a fish can see, is dependent on the amount of suspended material in the water. This material may be sediment (sand, mud etc.) or it may be plankton. There are a number of factors which influence water clarity. Tidal action is a major factor. For instance, a patch of clear water offshore can quickly become turbid as a receding tide brings out sand or mud. Similarly, an incoming tide may bring clear offshore water into cloudy nearshore areas. Currents are a major determinant of water clarity. For instance, the waters of El Niños tend to be clear. Upwelled water (water brought up from deep depths to the surface by winds) starts off very clear. However, this water contains huge amounts of nutrients and, within a few days, plankton starts to grow, which makes the water cloudy. Particularly in shallow water, winds and waves stir up the bottom, making the water cloudy. Similarly, periods of calm help the sand and mud to settle out, and water visibility increases. Sediment-carrying river water may cloud the ocean for miles.
Some fishes are attracted to cloudy water, while others tend to avoid it. Small species, such as white croakers, northern anchovies and sardines, often seek out turbid water because it helps protect them from predators. Lobsters are another species that defend themselves against predation by hiding in crevices by day and foraging by night; lobsters prefer to feed in muddy water. For the same reason, some predators, such as salmon and California halibut, may congregate in this water, attracted there by fishes on which they feed. However some species, particularly oceanic ones such as tunas and swordfishes, are usually found in clear waters. These are animals that depend heavily on vision for survival, and they are rarely found in cloudy nearshore waters.
Fisher folk are keenly aware of water clarity. A good example is salmon trollers. Fishermen know that salmon may be found in turbid water, called "coffee water" or "salmon water." Trollers will actively seek out these conditions to find fish. It is likely that salmon are attracted to "coffee water" because their foods, anchovies, sardines and other small fish, are hiding in this habitat. Tuna fishermen tend to avoid cloudy water and search for "tuna water", very clear, deep blue water. Gillnetters have found that turbid water is best for capturing their quarry (white seabass, California halibut, angels sharks, thresher sharks etc.). This may be because the nets are less visible in cloudy water. Also, small prey species are usually found in the turbid water, and their presence attracts the larger fish.
Water Motion - Water motion may be seen in a number of phenomena. Currents are large-scale horizontal movements of water caused by prevailing winds. Most currents are relatively slow and most are somewhat predictable. Swells and waves may move water about, usually more in a vertical dimension than in a horizontal one. If you observe a sea gull sitting on the water before, during and after a swell passes, you will see that while it goes up and down a number of feet, it really doesn't move along the water very much. Tides move great amounts of water inshore and offshore and, as mentioned before, the effects of tides can be seen in water many hundreds of feet down.
Overall, water movement may be the most important influence in a fish's life, primarily because it controls so many other critical factors. First, fishes have to be able to control their position. If water movement is too intense (say near a beach during a massive storm), many species will move out of the area, into calmer water. This is one of the reasons that smaller or medium-sized schooling fishes (such as white croakers, northern anchovies and Pacific sardines) tend to move away from very shallow water during the winter. They are at a disadvantage in turbulent conditions. By the same token, their predators (such as California halibut or white seabass) probably could withstand the rough, winter conditions, but they too move offshore following their food supply.
During the fall, lobsters are found in shallow water, often in depths of only a few feet. But as winter storms batter the coast, the water near shore becomes rough and lobsters move offshore. Lobster fishermen often start the season in the fall by setting their traps in shallow water, but they know that as the season progresses they will likely have to set them deeper.
On the other hand, some fishes seek out rough water because their food may be exposed or at a disadvantage there. California halibut sometimes congregate at the mouths of estuaries, where currents can momentarily disorient small fishes. A number of fishes inhabit the surf zone, right in the largest waves, where they pick
off those sand crabs that have been exposed by the surf.
Currents also can bring in or take away food, particularly for those fishes (such as Pacific herring and Pacific sardines) that eat plankton. Nutrient-rich upwelled water contains high concentrations of plankton and these fishes will concentrate in areas of upwelling. When the upwelling stops, this current and the plankton dissipate, and the fishes move on. An even more striking example is an El Niño current, which contains very little plankton and may bathe hundreds of miles of California coast. When this occurs, fishes may not find enough to eat, causing their growth to slow and their reproduction to be impaired. If plankton-eating fishes move out of an area or do not reproduce well due to lack of food, this in turn has a negative effect on larger predatory fishes.
Currents have a profound effect on reproduction. Many fishes spawn near the surface to take advantage of currents. First, the currents take the eggs or larvae away from the immediate vicinity, which is often a place where there are many organisms waiting to eat the newly-spawned animals. Second, the currents may carry the young to nursery areas, where more food is available. What happens is that currents vary between years, in speed and direction, and in some years the young are carried where they should be and in others they are not. Thus, some years produce lots of young fishes and other years do not.
Reef fishes often station themselves at the up-current side of the reef, in order to be the first predators to get a crack at whatever food is carried onto the reef by the current. Thus, often there will be a school of fishes on one end of the reef, but few on the other. In turn, the species which prey on these fishes may concentrate on the up-current end.
Water motion also influences both temperature and water clarity. Currents bring with them warmer or colder water and water of different clarity.
Tides An incoming tide may bring in clear offshore water, while an outgoing one may carry sand or mud and cause increased turbidity. Salmon trollers sometimes find that the salmon bite changes with the change of tide. Perhaps this is due to sudden changes in water clarity.
Fishermen pay close attention to water movement and often take advantage of the way that fishes respond to it. For instance, California scorpionfish are a popular food fish in southern California. Some fishermen have noted that, while this species normally lives on the bottom, during the night from June to September it predictably comes to the surface to spawn, probably to take advantage of the currents. These fishermen know the scorpionfish spawning grounds and fish for them at night, at the surface.
Fishermen are often keenly aware of the inshore-offshore movements of fishes and invertebrates and these movements are often associated with increased swells and turbidity inshore during winter. As mentioned before, winter storms seem to drive lobsters into deeper water, as well as small schooling fishes and their predators. Halibut fishermen may quit fishing in shallow water as soon as storm-driven heavy swells hit a beach. Not all fishes seem to be influenced by intense water motion, however. Fishermen report that swordfish will often stay right at the surface and catches will remain good even in very heavy seas and high winds.
Water salinity In most of the ocean, salinity is fairly constant. For this reason, most fishes have very little tolerance for changes in salinity. Only at the mouths of rivers and streams does salinity vary, and it varies with the time of year and distance from the mouth. The high water period, usually in winter and spring, brings with it increased runoff and increased water flow. In these circumstances, the freshwater plume is greater and extends to sea farther. In many circumstances, marine fishes are driven away from low salinity environments and fishermen may avoid them. However, there are some instances when the opposite is true. California halibut are often more abundant around river and stream mouths, and fishermen will often seek out these sites when pursuing halibut. The halibut are probably there because ocean waters around river mouths are typically cloudy from river sediment and stirred up bottoms. As mentioned before, cloudy water attracts white croakers, northern anchovies etc. and the halibut are attracted to these prey.
Light levels Light varies greatly in the ocean. First, light only penetrates a relatively short distance into the water. Below about thirty feet, red light does not penetrate and by about 400 feet all that remains of the color spectrum is green and blue. By 1,000 feet, even that light from the sun is gone. Second, the amount of light available to organisms is dependent on the turbidity of the water. Water clarity is discussed above, but basically the more material in the water (sand, mud, plankton etc.), the more light will be scattered or absorbed, and the darker the water will be. Third, moon phase is a major factor in influencing light levels; full moons produce considerable light, while new moons produce relatively dark nights. Light levels also change with time of day and with time of year.
Fishes are very sensitive to light and often link their behavior to light levels. Many fishes are most active during the crepuscular periods, that is at dawn and dusk. That is when they tend to feed most readily and often school most tightly. However, some species are diurnal (most active during the day) and some are nocturnal (active at night). Seasonal light level changes (lengthening light periods during spring and shortening periods during fall) are also one of the cues marine organisms use to begin migrations and to reproduce.
A knowledge of fish behavior as it is influenced by light levels is often crucial to successful fishing. For instance, during the night anchovies often swim about individually or in small groups. During this time they are not easy to catch in quantity by purse seine or lampara nets. However, just as light levels begin to increase with dawn, the fish school up and are most susceptible to capture. Fishermen know that this period may be quite limited, because when the sun comes up the anchovy schools may travel into the kelp beds or swim downward, both activities making them virtually impossible to catch.
Gillnetters have found that white seabass can often see a gillnet in the daylight and on very dark nights. On dark nights the bioluminescence in the water (produced by certain planktonic organisms) causes the net to glow and the seabass avoid it. However, when the moon is full, the moonlight counteracts the glow and helps decrease the net's visibility. Seabass also tend to be caught in gillnets more readily at dawn and dusk, probably because the fish are actively feeding then and do not notice the net. The periods around the new and, particularly, full moons seem to produce the best fishing in many fisheries. Fishermen may set more nets or spend more time fishing during these periods.
Opposite from seabass, swordfish catches are often lower during the full moon, peaking when nights are darkest. Salmon trollers sometimes catch fewer salmon during the days following full or nearly full moons, perhaps because the fish are feeding at night and are not hungry during the day.
Sometimes feeding fish are actually less susceptible to capture. For instance, California halibut draggers have found that, in general they catch more fish during the day, because halibut often feed at night. However, on those occasions when the fish feed during the day, night catches increase. Observant fishermen note these occurrences and alter their fishing efforts accordingly.