Fish eat all plant and animals materials, and the products of decay, but are primarily opportunistic carnivores. Most fish are euryphagous, but some are stenophagous. Few are monophagous. Food may be evasive or non-evasive.

Food is patchy in space and time. In time, there is natural periodicity on diurnal, monthly and seasonal cycles of temperature, light, tides, water availability, etc. In space, food abundance varies in horizontal and vertical (depth) dimensions. Patches are common; macrophyte patches, reefs, upwellings, etc. The photic zone where primary production occurs is a vertical patch.

Different locomotor patterns are used by fish feeding on food types, characterized in terms of their evasiveness and distribution (patchiness). Searching is the major problem on widely distributed food, with forms tending towards thunniform. Special behaviors are needed to capture elusive food. Capture specializations occur with locally abundant food, culminating in suction feeding replacing translocation. Specialists locomotor forms under-exploit smaller abundant items, which are taken by small generalist swimmers.

Fish usually strike from close to their prey, a few millimeters for larvae, and a few centimeters for musky. Fish must either wait for prey to approach to closely or stalk prey.

Body shape affects stalk and strike success. Round-bodied shapes have higher thresholds (prey respond later) compared to fish with elliptical shapes and pointed apices.

When at the limit of the approach distance after a stalk, or prey strays too near an ambushing predator, the predator fish strikes. Optimal strike tactics can be deduced from principles of Differential Interception Games.

• predators assume prey will respond, and in this fitness-critical situation, the response will be maximal (a startle response - fast-start).
• to maximize its chance of success, the predator should seek to minimize the duration of an interactions.
• to maximize its chance of success, prey should seek to maximize the duration of an interaction. Cover is typically within 1 to 3 meters away, as structures, school members, distance in the "gloom".

Therefore, the predator should:

• strike at the center of mass as a target.
• strike from the side to anticipate the likely escape trajectory.
• The predator should move as fast as possible, that is maximize their acceleration rate, also in a fast-start.
• The predator should also use S-starts rather than C-starts. Performance is improved when fish bend into a pre-strike S-posture.

Fish seek to strike before prey can adequately react, but this tends to reduce the ability to react to prey maneuvers. Therefore chases are not common.

Prey often disguise cues to the center of mass strike target and/or provide misdirection cues of false eyes and various color patterns.

Large Body Depth

Large body depth is an effective deterrent against gape-limited predators.

Spines

Schooling

The presence of conspecifics has a variety of prey-deterrent functions, confusing and deterring a predator, diluting the attack and providing group vigilance.

Attack threat is diluted by numbers.

Behavior

Move to keep the minimum approach distance of the predator as large as possible (fountain effect). Delay the startle response as long as possible as the bite offers a window for escape. During a chase, potentially slower prey can escape by turning with a smaller radius to enter the larger turning circle of the faster predator.

Food Capture

The earliest capture mechanism is ram feeding. Ventral and dorsal coupling to the hypaxial and epaxial myotomal muscle open the mouth. More recent elasmobranchs have the upper jaw connected to neurocranium via the hyomandibular. This increases mobility and allows a large gape and limited jaw protrusion. Among actinopterygians, the occurrence of the coronoid process, larger adductor muscles for the mandible, and freeing the maxilla (from chondrostean to holostean levels) increased torque around the jaw articulation. Starting with the Halecostomes, modifications occur in the head musculo-skeletal complex facilitating suction feeding, and later jaw protrusion.

Fish carry teeth on virtually every mouth and pharyngeal bone, including the gill rakers. The gill rakers protect the delicate gills, but also play a role in food treatment. Teeth architecture varies and there is some correlation with preferred diet.

The esophagus and stomach are often very distensible, as necessary when food is rarely broken into small pieces. The stomach may be associated with a gizzard. Cecae are often present to extend the time for digestion of plant material. The intestine varies in length according to diet. As a rule of thumb, the ratio body length/gut length is >3 in herbivores, 1 to 3 in omnivores, and <1 in carnivores.

Titers of various enzymes correlates with dominant diet items. Absorption efficiency (important in aquaculture) varies with diet. Similarly transporters vary among fish according to diet, with ontogenetic changes apparently being genetically determined. Animals that change diet normally during ontogeny often show ontogenetic changes in transporter activity.