Larval Fish

Larval fish (or ichthyoplankton) are a common, seasonal and potentially diverse component of the zooplankton of the majority of freshwater habitats. Compared with estuarine and marine fish species, only a limited number of identification guides are available for freshwater larvae (for example, Moser et al. 1984; Neira et al. 1998; Serafini and Humphries 2004).

Larval fish are often difficult to identify to the species level as they often have completely different morphological features to adults. As for estuarine fish, the most common method for identifying larvae is the series method or using existing keys and descriptions where they are available. The series method involves identifying the largest available larval or juvenile specimen, based or adult characteristics such as fin meristics and vertebral number (equivalent to the number of myomeres or muscle blocks - in larvae). The largest specimen is linked to smaller specimens in the series by using morphological and pigment characteristics. A variety of characters can be used to identify fish larvae including their general morphology such as the body shape and gut length and degree of coiling, the number of myomeres, pigmentation patterns (melanophores), the sequence of development of fins and the pattern of head spination (Table 7.2, Figure 7.1). The length and stage of development are important features in identification of larvae. For example the stage of flexion is when the notochord begins to grow upward (dorsally) and the bony structures of the tail fin begin to form on the ventral surface. Compared with the larvae of estuarine and marine fish, larvae of many freshwater species have a large yolk sac and morphological changes such as notochord flexion and development of the fin elements occurs at a larger size.

Most freshwater fishes have seasonal reproduction, with peaks in reproduction, and therefore larval abundance, generally occurring in spring and summer (Wooton 1998). Some species spawn over a relatively long time period (months), while others spawn period very briefly (a few days) (Matthews 1998). Therefore, the potential species composition of the ichthyoplankton is likely to change considerably from one sampling time to the next.

Larval fish can be found in rivers, creeks, lakes, reservoirs, off-channel habitats such as billabongs (ox bow lakes), wetlands and even in temporarily inundated habitats such as floodplains and ephemeral creeks. Larvae use a variety of habitat patches in freshwater systems, such as open water (pelagic) habitats, complex submerged macrophytes and woody debris, interstitial spaces of gravels, littoral habitats and backwaters. Some species also have fairly specific requirements at certain developmental stages; for example, many cyprinids have a downstream drifting dispersal phase, while other species require parental care in protected nest areas, such as in hollow logs.

The early life of fishes - from embryo to larvae to juveniles - is marked by rapid changes in morphology, ecology, growth and behaviour (Fuiman and Higgs 1997; Trippel and Chambers 1997). These changes often result in

Table 7.2. Freshwater fish larval characteristics (modified from Neira et al. 1998 and Serafini and Humphries 2004).

Family

Features

Eleotridae (gudgeon)

28-34 myomeres; body elongate; lightly pigmented; gut moderate and slightly coiled; conspicuous gas bladder; demersal eggs

Atherinidae (hardyhead)

34-36 myomeres; body very elongate; moderately pigmented; gut coiled and compact; demersal eggs

Cyprinidae (carp)

36-40 myomeres; body elongate; moderate to heavily pigmented; gut long and straight; demersal eggs

Gadopsidae (river blackfish)

49-50 myomeres; lightly pigment until late postflexion; large yolk sac; gut moderate to long and straight; demersal eggs

Galaxiidae (whitebait)

36-64 myomeres; body very elongate; lightly to heavily pigmented; gut long to very long and straight; demersal eggs

Melanotaenidae (rainbow fish)

34 myomeres; body elongate; moderately pigmented; gut short and coiled; demersal eggs

Retropinnidae (smelt)

45-53 myomeres; body very elongate; lightly pigmented; gut very long and straight; demersal eggs

Percichthyidae

(cod/pigmy perch)

27-36 myomeres; body elongate to moderate; moderate to heavily pigmented; gut moderate to long and loosely coiled; large yolk sac in some genera; weak preopercular spines; demersal eggs

Plotosidae (catfish)

>77 myomeres; body elongate; moderately to heavily pigmented; gut moderate and loosely coiled; mouth barbells; large yolk sac; demersal eggs

Poeciliidae (Gambusia, mosquito fish)

31-33 myomeres; body moderate; moderately pigmented; gut short and coiled; live bearer

Percidae (redfin)

39-41 myomeres; body elongate; lightly pigmented; gut moderate and loosely coiled; conspicuous gas bladder; demersal eggs

Terapontidae (silver perch/ grunter)

25 myomeres; body elongate; lightly pigmented; gut coiled and moderate; small preopercular spines; demersal eggs

dramatic changes in habitat and diet use within a species. For example, some riverine fishes are exclusively found in shallow, still, off-channel habitats as newly hatched larvae, but then move to a variety of mid-channel habitats as older larvae and juveniles (see, for example, Scheimer and Spindler 1989). Similarly in lakes, many species occur in structurally dense, shallow, littoral habitats as small larvae and then move to mid water, deeper habitats as larger individuals. Movements of larval fish can also occur vertically, with diel migrations between surface waters and benthic habitats being common, particularly in deeper environments.

Larval fishes are a useful and sensitive tool for monitoring the effects of various anthropogenic influences on the system. For example, the presence of fish

Figure 7.1 Outlines of larvae of some typical freshwater fish families approaching flexion. a) Percichthyidae (cod), b) Percichthyidae (pigmy perch), c) Melanotaenidae (rainbow fish), d) Cyprindae (carp), e) Terapontidae (silver perch/grunter), f) Percidae (redfin), g) Poecliidae (Gambusia, mosquito fish), h) Eleotridae (gudgeon), i) Atherinidae (hardyhead), j) Galaxiidae (whitebait), k) Retropinnidae (smelt), l) Plotosidae (catfish), m) Gadopsidae (river black-fish). Scale bar is 1 mm. (Modified from Neira et al. 1998 and Serafini and Humphries 2004.)

Figure 7.1 Outlines of larvae of some typical freshwater fish families approaching flexion. a) Percichthyidae (cod), b) Percichthyidae (pigmy perch), c) Melanotaenidae (rainbow fish), d) Cyprindae (carp), e) Terapontidae (silver perch/grunter), f) Percidae (redfin), g) Poecliidae (Gambusia, mosquito fish), h) Eleotridae (gudgeon), i) Atherinidae (hardyhead), j) Galaxiidae (whitebait), k) Retropinnidae (smelt), l) Plotosidae (catfish), m) Gadopsidae (river black-fish). Scale bar is 1 mm. (Modified from Neira et al. 1998 and Serafini and Humphries 2004.)

larvae clearly indicates that fish have spawned recently, and this can be used to elucidate the success of particular rehabilitation strategies targeted to enhance spawning, such as environmental flows (Humphries and Lake 2000).

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