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University of California Press
Inland Fishes of California / Edition 2

Inland Fishes of California / Edition 2


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Product Details

ISBN-13: 9780520227545
Publisher: University of California Press
Publication date: 05/21/2002
Edition description: First Edition, Revised and Expanded
Pages: 517
Product dimensions: 8.50(w) x 11.00(h) x 1.60(d)

About the Author

Peter B. Moyle is Professor of Fish Biology at the University of California, Davis; author of Fish: An Enthusiast's Guide (California 1993); and coauthor of Fishes: An
Introduction to Ichthyology
(4th edition, 2000), Methods for Fish Biology (1990), and Distribution and Ecology of Stream Fishes of the Sacramento—San Joaquin Drainage System, California (1982).

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University of California

Copyright © 2002 Regents of the University of California
All right reserved.

ISBN: 0-520-22754-9

Chapter One

Herrings, Clupeidae

If sheer number of individuals is the criterion for success, herrings are one of the most successful families of fishes in the world. Early in the history of teleost evolution, they achieved plankton-feeding specializations that have allowed them to remain abundant. Herrings have highly protractile jaws and long, fine gill rakers for picking and filtering plankton. Their scales are cycloid, deciduous, and silvery, reflecting light like miniature mirrors to confuse predators. Their bodies are muscular yet deepened by a sharp keel on the belly. The keel eliminates the faint belly shadow most fishes have when seen from below, thus increasing the difficulty predators have in picking out individuals from a shoal. Indeed, most morphological specializations of the Clupeidae enable them to function in the huge schools in which they are typically found.

Although usually thought of as marine, herrings are also successful as anadromous and freshwater fishes. Thanks to humans, the ranges of some forms have been greatly extended, especially in North America, with results ranging from beneficial to disastrous. Small freshwater shads of the genus Dorosoma have long been regarded as ideal forage fish in reservoirs and large lakes, and consequently have been distributed throughout the United States. Unfortunately, most of these introductions took place before it was realized that plankton-feeding fishes can alter lake ecosystems by changing the zooplankton community, causing clear lakes to become green with algae and reducing the amount of food available to the young of game fishes. Often shad introduced as forage fish cause declines in the very predatory fishes whose populations they were supposed to enhance (DeVries and Stein 1990).

Only two clupeid species are regularly found in California's fresh waters, threadfin shad and American shad. Both were introduced with greater success than was perhaps ever imagined. In addition, Pacific herring (Clupea harengeus) occasionally wander into fresh water when they move into estuaries to spawn. Northern anchovy (Engraulis mordax), in the closely related family Engraulidae, wander into brackish water on occasion, usually as juveniles.

Threadfin Shad, Dorosoma petenense (Günther)

Identification Threadfin shad are small (rarely longer than 10 cm TL in California) with the typical deciduous scales, flattened bodies, and sawtooth bellies of most herrings. They are distinguished by the long, threadlike final ray of the dorsal fin and by the single dark spot behind the operculum. The mouth is oblique, small, and toothless. The upper jaw is longer than the lower. The dorsal fin (11-17 rays, usually 14-15) is falcate. Anal fin rays number 17-27; scales in the lateral series, 40-48; belly scutes (scales), 15-18 before the bases of the pelvic fins and 8-12 behind them. The intestine is long and convoluted, with a gizzardlike stomach. The gill covers are smooth or have a few faint striations. The overall color is silvery, although the back frequently has a black or bluish hue.

Taxonomy The subspecies of threadfin shad introduced into California is D. p. atchafalayae (after the Atchafalaya River, Louisiana), although subspecies designations may have little validity (1).

Names Doro-soma means lance-body, referring to the eel-like larvae. Petenense is after Lake Petén, Guatemala, from which the first specimens were described. Threadfin refers to the distinctive dorsal fin. Shad is apparently derived from the ancient Celtic name for herring.

Distribution Threadfin shad are native to streams flowing into the Gulf of Mexico, south to Belize. In the Mississippi River and its tributaries they are found as far north as southern Indiana and Illinois (1). Shad from the Tennessee River at Watts Bar, Tennessee, were introduced into ponds in San Diego County in 1953 by CDFG (2). In 1954 fish from these ponds were introduced into San Vincente Reservoir, San Diego County, and Havasu Reservoir on the Colorado River. About 1,000 shad were planted in Havasu, and within a year they numbered in the millions and quickly spread downstream (2). In subsequent years they were planted by CDFG in reservoirs throughout the state, with the Sacramento-San Joaquin drainage planted in 1959 (3). From these transplants they have become established in the Sacramento-San Joaquin River system and its estuary, as well as in most of the lower Colorado River and the canals and drains of the Salton Sea region. Unauthorized plants have further expanded the range to more isolated lakes and reservoirs, such as Pillsbury Reservoir and Clear Lake, Lake County. They are occasionally taken in salt water from Long Beach to Yaquina Bay, Oregon (1, 4). Besides California, threadfin shad have been planted successfully in suitable waters throughout much of the United States, including Hawaii.

Life History Threadfin shad inhabit open waters of reservoirs, lakes, and large ponds as well as sluggish backwaters of rivers. In reservoirs they often congregate near inlets of small streams or along steep surfaces of dams. They prefer well-lighted surface waters and are seldom found below depths of 18 m (3). The best growth and survival occur in waters in which summer temperatures exceed 22-24°C and that do not become colder than 7-9°C in winter. Threadfin shad apparently cannot withstand water colder than 4°C for long (5). A sudden drop in temperature also causes high mortalities, as do prolonged periods of cold water. The population in the Sacramento-San Joaquin Delta experiences heavy die-offs every winter when the water cools to 6-8°C (6). In Clear Lake threadfin shad became abundant in 1985, but they were apparently extirpated during the exceptionally cold winter of 1990-1991. They reappeared in 1997, perhaps as the result of an illegal reintroduction (18).

Threadfin shad live mainly in fresh water and become progressively less abundant as salinity increases. Nevertheless, they can survive and grow in sea water (4). Salt water apparently inhibits reproduction: shad in the Salton Sea have failed to reproduce despite continuous recruitment from inflowing canals (7).

Threadfin shad form schools segregated by size and hence by age. Although shad concentrate in surface waters, young-of-year tend to be found in deeper water than adults, especially at night (8). When attacked by predatory fish such as striped bass, a school of shad will close together and hug the water surface, with some individuals leaping from the water at each attack. At such times they also are vulnerable to terns and other fish-eating birds.

Like all clupeids, threadfin shad are plankton feeders. They use their gill rakers to strain small (<1 mm) zooplankton, phytoplankton, and detritus particles from the water but feed individually on larger organisms, mostly zooplankton (9). This ability to feed by both filtering and picking allows for broad diets. Planktonic organisms often occur in their digestive tracts in roughly the same proportion as in the water during the day. At night, however, phytoplankton and detritus predominate in the stomachs, because low light levels restrict the ability of shad to feed by picking (9). Although shad can grow and reproduce on filtered prey alone, larger zooplankters, such as cladocerans and copepods, appear to be preferred (19). Large shad populations can virtually eliminate larger zooplankton species from lakes (10).

Threadfin shad are fast growing but short lived. Under optimal conditions they can increase in length 1-3 cm per month during the first summer of life, reaching 10-13 cm TL by the end of summer. Normally they reach only 4-6 cm TL by the end of their first year and 6-10 cm TL by the end of their second. Few live longer than 2 years or achieve more than 10 cm TL, although rare individuals may live as long as 4 years (8) and achieve lengths of 33 cm TL (11). The largest threadfin shad recorded from California was 22 cm TL, from the Salton Sea (7). Fish from nonreproducing saltwater populations frequently achieve larger sizes than are normal for fresh water (8). In fresh water one of the main factors limiting growth seems to be interspecific competition for food and space (8). Thus shad newly established in reservoirs tend to grow larger during their first year than their counterparts in more established populations.

Threadfin shad may spawn at the end of their first summer but usually wait until their second (8). Spawning takes place in California in April through August, peaking in June and July when water temperatures exceed 20°C (12). Spawning, however, has been observed at 14-18°C (13). Despite a protracted spawning period, each shad apparently spawns once a summer (8). Spawning is most often at dawn and centers around floating or partially submerged objects, such as logs, brush, aquatic plants, and gill nets. Small, compact groups of shad swimming close to the surface approach such objects rapidly, turning away just prior to collision. As they turn, eggs and sperm are released (14). The fertilized eggs stick to surfaces. Spawning is usually accompanied by splashing and leaping from the water. Females produce 900 to 21,000 eggs, the number increasing sharply with the size of the fish (8). The ability of threadfin shad to deposit eggs on floating objects may help account for their success in reservoirs, in which fluctuating water levels frequently expose embryos attached to fixed objects (12).

The embryos hatch in 3-6 days and larvae immediately assume a planktonic existence. They are weak swimmers and so are susceptible to entrainment in water diversions and power plant intakes. However, they are capable of some vertical migration, preferring surface waters during the day and deeper waters at night (12). The length of the planktonic life stage is not known exactly, but it is probably 2-3 weeks depending on temperature. Larvae metamorphose into juveniles at about 2 cm TL. Juveniles form dense schools and, in estuaries, are found in water of all salinities, although they are most abundant in fresh water (12).

Status IIE. Threadfin shad were brought into California in 1953 on the optimistic assumption that they were ideal forage fish for reservoirs that should greatly improve the growth rates of game fishes. Their desirability stemmed from their small size, high reproductive rate, and ability to occupy open waters presumed to be unexploited fish habitat. In many reservoirs managed for trout, especially those receiving large plants of catchable-size fish, the growth of trout larger than 28 cm FL can be extremely rapid when they feed on shad. The success of striped bass in Millerton Reservoir, Fresno and Madera Counties, is probably due largely to their diet of shad. In other reservoirs, large-size largemouth bass, black and white crappie, and white catfish also utilize threadfin shad. Unfortunately, shad are largely unavailable to small warmwater game fishes. Because the young of many centrarchids live in open water for extended periods of time, feeding on plankton, shad may actually compete with them by reducing plankton populations. In particular, threadfin shad may eliminate large species of planktonic crustaceans important in the diets of larval fishes (10). Thus, in some California reservoirs, growth and survival of young centrarchids, including largemouth bass, decreased after introduction of shad (15). In Clear Lake the establishment of shad was followed by the crash of Daphnia populations, decreased survival of juvenile largemouth bass, and huge increases in the numbers of piscivorous birds (e.g., western grebes, double-crested cormorants, white pelicans) (18). When the shad population collapsed in 1990, bass, bird, and zooplankton populations returned to pre-introduction levels. When shad became reestablished (1998), Daphnia collapsed again and the birds returned (18). The increase was also accompanied by a huge die-off of shad in 1999, littering beaches with dead fish.

Mixed success with shad introductions has been experienced elsewhere, leading to attempts to eradicate them from some waters (16, 17). Unfortunately, although fisheries managers may have adopted a more cautious attitude toward threadfin shad introductions, enthusiasm for this fish still persists among some anglers, resulting in unauthorized introductions.

The ability of shad populations to increase explosively has allowed them to spread rapidly by "natural" means far beyond original introduction sites. The thousand shad introduced into Havasu Reservoir managed to provide enough offspring to colonize the entire lower Colorado River and Salton Sea basin in less than 18 months (3). In the Sacramento-San Joaquin drainage they quickly established populations downstream from reservoirs and then spread throughout the California Aqueduct system. In most areas they colonized, their effect-especially on native fishes with planktonic larvae and on young centrarchids-is unknown. In the Sacramento-San Joaquin Delta shad are a major item in the diet of striped bass and other piscivorous fishes, but their role in the ecosystem is poorly understood. Numbers in the Delta have gradually declined since the late 1970s, reflecting a general decline of planktonic fishes in the estuary.

American Shad, Alosa sapidissima (Wilson)

Identification American shad are large (to 75 cm FL) clupeids with thin, deciduous scales, compressed bodies, and a sawtooth keel on their bellies. The mouth is terminal, and the upper and lower jaws are about equal in size, the lower fitting into the central notch of the upper (but sometimes projecting slightly). The dorsal fin (15-19 rays, usually 17-18) is short and straight edged, without the greatly elongated last ray characteristic of threadfin shad. Anal fin rays number 18-24 (usually 20-22); scales in the lateral series, about 50-55 (the lateral line is poorly developed); belly scutes (scales), 19-23 (usually 22-23) before the bases of the pelvic fins and 12-19 (usually 15-17) behind them (1). Gill rakers are long and slender, with 59-73 below the sharp bend on the first arch. The opercula have coarse, fanlike striations on their surfaces. Live fish tend to be steely blue on the back and silvery on the sides. They are distinguished by a row of 4-6 black spots that starts on the back just behind the operculum. The first spot is larger than the rest.

Taxonomy American shad show biochemical, meristic, and life history differences among watersheds in their native range along the Atlantic coast, but these differences are clinal, so no subspecies are recognized (10, 11, 24).


Excerpted from INLAND FISHES OF CALIFORNIA by PETER B. MOYLE Copyright © 2002 by Regents of the University of California. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents

Conversion factors
Distribution patterns
A conservation strategy
Key to the Inland Fishes of California
Bullhead catfishes,Ictaluridae
Salmon and trout,Salmonidae
Color plates follow page
Striped basses,Moronidae


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