Just as birders have their field guides, we diving naturalists have access to a wide variety of excellent fish ID guides. Unfortunately, most of us don't know how to use them. A field guide is a tool and, much like any other tool, there is a proper way to use it and dozens of improper ways. Fortunately, the worst result of improper use of a field guide is usually nothing more serious than an incorrect identification. But if you're serious about putting names to the fish you see while diving, it pays to learn how to use each guide properly. A field guide is a lot like a cook book or auto repair manual: it's a set of instructions, prepared by an expert, on how to accomplish a specific task - be it bake a flan, replace an air filter, or identify a fish - with the minimum likelihood of error.
One characteristic common to most expert naturalists is the development of idiosyncratic 'short cuts' or 'tricks' to help them identify creatures. Such 'tricks' might include categorising creatures by shape, colour, or habitat. When writing their books, most field guide authors fuse aspects of their own system into the traditional scheme of scientific classification. As a result, each field guide is different in how it is intended to be used. It is thus always a good idea to read the introductory sections of any field guide, paying particular attention to any section called 'How to Use This Book'. This simple expedient can save you a lot of time and error when using the book.
But for all their idiosyncracies, most field guides use standard scientific terms and follow scientific classifications quite closely. Skim through almost any field guide and you're likely to see an awful lot of unfamiliar, tough-to-pronounce, and altogether formidable-looking words - often in italics. Believe it or not, these terms are a form of short-cut to aid identification. But they save time only if you're familiar with the terms used, and know something about the standard system of biological classification. The anatomical and oceanographic terms you'll need to know when using a field guide will almost always be defined in either the introduction or the glossary (if there is one). To learn all these terms may seem difficult at first, but most of them will hold you in good stead from field guide to field guide. Invest a little time in learning these terms now; save yourself a lot of time and mistakes later.
The standard system of biological classification is the same one you learned in high school biology: kingdom, phylum, class, order, family, genus, and species. The good news is, you don't need to know the precise taxon of any given creature for this system to be useful in aiding identification. Taxonomy is more than a series of arbitrary categories: it reflects the probable evolutionary sequence of the creatures described. Think of biological classification as a kind of filing system, becoming more precise and more closely related from kingdom (the broadest category) down to species (the narrowest). In this analogy, kingdom is like a group of filing cabinets filing loosely-related materials, phylum like an individual cabinet with more closely-related material, class a particular drawer, order a specific file, family a document, genus a chapter within a document, and finally species an individual paragraph. The beauty of this system is that even if you cannot identify a given creature all the way down to species, a knowledge of biological classification can at least tell you what drawer to look in. Because almost all field guides use this same system of classification, this knowledge can direct you to the appropriate section of your field guide.
However, some levels of classificatory resolution are - according to the Goldilocks school of allegory - too broad, some too narrow, and another 'just right'. But which level of resolution is that? Kingdom is too broad. Ditto phylum and class. But species is obviously too narrow - if you knew all the species, you wouldn't need a field guide. I would suggest that family is the optimal level of resolution to begin learning fish identification. Members of a given family look sufficiently alike to enable the diving naturalist to learn to recognise features that characterise them. Some families may have members that look superficially like members of another, but once you know what characterises the more common families you'll be much less likely to be deceived. If you take the time to learn to tell such look-alikes as angelfishes (family Pomacanthidae) from butterflyfishes (Chaetodontidae), snappers (Lutjanidae) from grunts (Haemulidae), blennies (Bleniidae) from gobies (Gobiidae), and wrasses (Labridae) from parrotfishes (Scaridae), you'd be well on your way. For example, the surest way to distinguish an angelfish from a butterflyfish is not size, as many believe, but the fact that the former have preopercular spines while the latter do not.
So how does one acquire such knowledge? The best technique I've discovered is to invest some time reading the familial accounts in your field guides and look for their diagnostic characters by analysing representative species within each family. Study those beautiful, full colour illustrations in light of the anatomical features you have learned. Better yet, make point-form notes of the one, two, or at most three most diagnostic characters of each family and make sketches of a few typical species. Label the hallmarks identifying each fish as belonging to such-and-such family right on your drawing (just like Peterson does for birds in his guides). Don't worry that you're not an artist; if you can write your name, you can draw. All you need is a basic knowledge of fish anatomy, a knowledge of field marks that characterise each family, and a place to start.
When drawing fishes, I recommend beginning with the simple alpha ( a ) fish shape you've known and used to represent fishdom since preschool. If the fish you are studying is long and slender or short and deep-bodied, simply draw your alpha accordingly. Then fill in the mouth, eyes, all the fins - taking note of their shape and position (Is the mouth upturned or straight, eye big or small, tail forked or squared off?). Finally, fill in any obvious body markings, using the anatomical features you've just completed to help get their placement and size just right. You'll be amazed at how easy it is to turn out good likenesses of various fishes. Once you're good at working from pictures in books, practice drawing live fishes at your local aquarium or pet store.
In the process of applying these skills, you'll teach yourself how to recognise members of various families and internalise the eye to mind co-ordination that will greatly enhance your observational skills. But you'll also develop something much more useful: the ability to sketch any unfamiliar fish you might see on any dive you make, anywhere in the world. Just a few practiced squiggles on your underwater slate will aid enormously your attempts to identify each 'new' fish when referring to your field guides back on deck or at home. Eventually, you'll be able to identify thousands of common fishes on sight, and have all the skills you need to identify the occasional rarity. And THAT'S how to get the most out of your fish ID guides.
Three of My Favourite Fish ID Field Guides:
Collins Pocket Giude: Coral Reef Fishes - Indo-Pacific & Caribbean. By Ewald Lieske and Robert Mysers. Harper Collins, 1994. [Comprehensive, up-to-date, well illustrated, and including numerous notes on characteristics of each family, colour morphs of individuals, reproductive and feeding biology of some of the better-known species; this is probably the best single-volume field guide to coral reef fishes available.]
Field Guide to Pacific Coast Fishes of North America. By William N. Eschmeyer, Earl S. Herald, and Howard Hammann. Houghton Mifflin, 1983. [Part of the Peterson Field Guide Series, this reasonably up-to-date book features excellent illustrations - many in full-colour - with identifying characters marked on the plates; good notes on the various families covered.]
Divers and Snorkelers Guide to Fishes and Sea Life of the Caribbean, Florida, Bahamas and Bermuda. By F. Joseph Stokes. Academy of Natural Sciences of Philadelphia, 1984. [The invertebrate sea life is poorly represented, but each fish species is illustrated in colour, often using several individuals to show different colour morphs; a bit dated in parts, but reasonably comprehensive and generally a very user-friendly field guide.