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Even for gelatinous species, siphonophores are exceptionally difficult to enumerate owing to their delicate colonial structure. Nets tend to reduce them to fragments; those fragments are difficult to quantify as numbers of individuals. The most appropriate techniques for evaluating numbers of siphonophores are mainly visual counts, either from diver‐based observations or for deeper‐living species, using submersible‐based observations either manned or unmanned (Remotely Operated Vehicles – ROV’s, or AUV’s – Autonomous Underwater Vehicles), Techniques include using diver‐powered meter hoops and flowmeters, counting individuals as they passed through the hoops (Purcell 1981a, b) and, in a variation of the same theme, using larger (5 m × 5 m) grids towed behind a slowly moving boat while divers count ( Biggs et al. 1981, 1984). Options using submersibles mainly include evaluating nearest‐neighbor distances (Mackie and Mills 1983) and mounting a hoop in the front of the submersible.

In the open ocean, siphonophores are found at densities of less than 1/1000 m³. However, they can number 5–10 m⁻³ (Table 3.7) in more productive coastal regions (Mackie et al. 1987). Most often they are outnumbered in the open ocean by ctenophores, but their coastal numbers can exceed those of other gelatinous forms.