question: Why this shoal of tiny fish surrounded and followed me while I was diving?
comment: Likely they were looking to school around you as protection from predators. By being around a larger creature they may disguise the shoal as one big creature, or at least prevent a predator from swooping up and eating a small fish for fear of involving the larger animal. There may have been a big shark or similar fish lurking around which you didn't see.
passages: In 2009, building on recent advances in acoustic imaging, a group of MIT researchers observed for "the first time the formation and subsequent migration of a huge shoal of fish." The results provide the first field confirmation of general theories about how large groups behave, from locust swarms to bird flocks.
   BULLET::::- Shoal size – The number of fish in the shoal. A remote sensing technique has been used near the edge of the continental shelf off the east coast of North America to take images of fish shoals. The shoals – most likely made up of Atlantic herring, scup, hake, and black sea bass – were said to contain “tens of millions” of fish and stretched for “many kilometers”.
   Section::::Reception.
   The most common incident regarding humans is strongly connected with "T. draco"’ s typical behaviour: being burrowed in the sand of shallow waters. Especially in the summer it is quite possible for a careless bather to step, jump or fall on the venomous spines of the greater weever. The other occasion where humans are at risk to get stung by "T. draco" is when fishermen are handling the fish if caught in a net.⁠⁠
   Multiple male Tennessee dace have been observed following a single female through pool and run areas. In fact, as many as 20 have been observed following a single female. Males would form a straight line with the snout of one male just behind the caudal fin of the preceding male. They have also been observed leaping out of the water one hour before and after spawning.
   Zhuo said he had seen the six seal-like, finned creatures swimming and frolicking in the lake for an hour and a half, before they disappeared around 7:00 a.m. "They could swim as fast as yachts and at times they would all disappear in the water. It was impressive to see them all acting at exactly the same pace, as if someone was giving orders," he said. "Their fins—or maybe wings—were longer than their bodies."
  
  Section::::In popular culture.
   BULLET::::- Density – The density of a fish shoal is the number of fish divided by the volume occupied by the shoal. Density is not necessarily a constant throughout the group. Fish in schools typically have a density of about one fish per cube of body length.
   In 2008, a shoal of 17 hadal snailfish, a species of deep water snailfish, was filmed by a UK-Japan team using remote operated landers at depths of in the Japan Trench in the Pacific. The fish were 30 centimetres long (12 in), and were darting about, using vibration sensors on their nose to catch shrimps. The team also reported that the appearance of the fish, unlike that of most deep sea fish, was surprisingly "cute", and that they were surprised by how active the fish were at these depths.
  
  Section::::Demersal fisheries.
   BULLET::::- Wootton, RJ (1998) "Ecology of Teleost Fishes" Springer.
  
  Section::::External links.
  
  BULLET::::- Collective Animal Behavior website organized around David Sumpter's book (2008) by the same name
  
  BULLET::::- STARFLAG project: Description of starling flocking project
  
  BULLET::::- Center for Biologically Inspired Design at Georgia Tech
  
  BULLET::::- David Sumpter's research website
  
  BULLET::::- Iain Couzin's research website
  
  BULLET::::- Website of Julia Parrish, an animal aggregation researcher
  
  BULLET::::- Pelagic Fisheries Research Program (2002) "Current status and new directions for studying schooling and aggregation behavior of pelagic fish"
  
  BULLET::::- Clover, Charles (2008) "Fish can count to four – but no higher" Telegraph Media Group.
   A frequent hydrodynamic stimulus is a wake, consisting of eddies and vortices that an organism leaves behind as it swims, affected by the animal's size, swimming pattern, and speed. Although the strength of a wake decreases over time as it moves away from its source, vortex structure of a goldfish's wake can remain for about thirty seconds, and increased water velocity can be detected several minutes after production.
  
  Section::::Overview of hydrodynamic stimuli.:Uses of hydrodynamic stimuli.
   The fish is generally wary of scuba divers; it will move slowly away as divers approach. However, schools have been known to crowd around divers, apparently attracted to the bubbles the diver exhales.
   Section::::Behavior.:Shoaling behavior.:Predator protection.
  
  In the presence of a predatory stimulus, shoal size increases to allow for greater protection via the dilution effect. From the dilution effect, each individual will have a lesser chance of being attacked by a predator, since there are many other individuals in the group. In a larger group, there is a larger chance that a predator will attack a different shoalmate than it will attack the individual of interest. Individuals who stray from the shoal have an increased mortality risk. This leads to selection for grouping.
   These sometimes immense gatherings fuel the ocean food web. Most forage fish are pelagic fish, which means they form their schools in open water, and not on or near the bottom (demersal fish). Forage fish are short-lived, and go mostly unnoticed by humans. The predators are keenly focused on the shoals, acutely aware of their numbers and whereabouts, and make migrations themselves, often in schools of their own, that can span thousands of miles to connect with, or stay connected with them.
   Fishing charters most often look for floating debris and frigatebirds near the edge of the reef in about of water. Mahi-mahi (and many other fish) often swim near debris such as floating wood, five-gallon bucket lids, palm trees and fronds, or sargasso weed lines and around fish buoys. Frigatebirds search for food accompanying the debris or sargasso. Experienced fishing guides can tell what species are likely around the debris by the birds' behavior.
   Typical vertical behaviour of bigeye tuna shifts when fish associate with seamounts, buoys and fish aggregating devices, with individuals remaining in surface waters. Association with objects has been observed to occur over periods of approximately 10–30 days. This associative behaviour of bigeye tuna (and also other species of tuna) is taken advantage of by fisheries with approximately 27% of all catches of tunas by purse seine vessels in the western and central Pacific Ocean derived from fish aggregating devices.
  
  Section::::Behavior.:Migration.
   Lancetfishes have been caught on longlines as shallow as ten fathoms in Oregon and the Gulf of Mexico. Some anecdotal reports have observed shoals of 30 - 40 individuals at the surface in Icelandic waters during spring. Hook and line capture of lancetfish from surf zones is not unheard of and dietary surveys suggest at least some feeding occurs within inshore waters. However, lancetfish are generally considered solitary, mesopelagic, and bathypelagic fishes occupying depths between 100 and 2000 m. While they have not been shown to participate in diel vertical migration, they have been found in a huge variety of depths.
   Section::::Diet.
  
  Although not much is known about its biology, the bayou topminnow is presumed to feed near the surface, where it consumes drifting organic matter, insects, and other animals associated with the water surface.
  
  Section::::Habitat.
  
  Bayou topminnows occur in and around shoreline vegetation of clear lakes and ponds, backwaters, and overflow pools of large rivers.
  
  Section::::Reproduction and life cycle.
  
  As with other topminnows, this species presumably has a spawning season that peaks in May or June, more protracted seasons may occur in some areas. Larvae of this species has not yet been described.
  
  Section::::Distribution.
   In 2005, near Monterey, California, at 1474 metres depth, an ROV filmed a female ceratioid anglerfish of the genus "Oneirodes" for 24 minutes. When approached, the fish retreated rapidly, but in 74% of the video footage, it drifted passively, oriented at any angle. When advancing, it swam intermittently at a speed of 0.24 body lengths per second, beating its pectoral fins in-phase. The lethargic behaviour of this ambush predator is suited to the energy-poor environment of the deep sea.
   Some shoals engage in mobbing behaviour. For example, bluegills form large nesting colonies and sometimes attack snapping turtles. This may function to advertise their presence, drive the predator from the area, or aid in cultural transmission of predator recognition.
   Besides the dilution effect, shoaling also benefits the individual because of group vigilance. To best protect against predation, the banded killifish must be on the lookout to detect predators. In a group, each individual can spend less time looking out for predators because others in the shoal can share in this responsibility. With increasing shoal size, each individual spends less time being vigilant, but with more individuals the group increases overall vigilance and therefore benefit each individual in the shoal. When one individual detects a predator, an alarm call is made to alert the shoal, so that the individuals can act to protect themselves from predatory attack.
   BULLET::::- Tagging
  
  Stelephant has been involved with the University of California, Santa Cruz Long Marine Lab researchers, and was tagged last spring to track his foraging habits while swimming and diving off the Aleutian Islands in the northern Pacific Ocean.
  
  BULLET::::- Current
   The main predators of "Megalops" during stage one and early stage-two development are other fish, depending on their size. Juveniles are subject to predation by other juvenile "Megalops" and piscivorous birds. They are especially vulnerable to birds when they come to the surface for air, due to the rolling manner in which they move to take in air, as well as the silver scales lining their sides. Adults occasionally fall prey to sharks, porpoises, crocodiles and alligators.
  
  Section::::Physical characteristics.:Swim bladder.
   A study using sonar in French Polynesia, found large shoals of juvenile bigeye tuna and yellowfin tuna aggregated closest to the devices, 10 to 50 m. Further out, 50 to 150 m, was a less dense group of larger yellowfin and albacore tuna. Yet further out, to 500 m, was a dispersed group of various large adult tuna. The distribution and density of these groups was variable and overlapped. The FADs were also used by other fish, and the aggregations dispersed when it was dark.
   In a public speech at the celebration of "Hoover Day" in the county seat of Madison, on August 17, 1929, President Hoover spoke of fishing and his camp:
  
  Fishing was conducted only outside the camp grounds, and the fish within camp were "so tame that if you threw a pebble in the water they will rush out at it, or perhaps drift slowly into the open to look you over."
   "Mnemiopsis leidyi" was first recorded in the Mediterranean Sea in 1990 and in 2009, large swarms were present in some areas. "Beroe cucumis" is native to the northern Atlantic Ocean and sometimes occurs in the Mediterranean Sea, and it is being monitored to see if it can provide some level of control of "M. leidyi". In 2012, "Beroe cucumis" was found off the coast of Israel for the first time, and there is proof that it preys on "M. leidyi", as an individual was found with a partially digested "M. leidyi" in its stomach.
  