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Maturity and fecundity in the rockfishes, sebastes spp., a review.

Introduction

Sebastes (rockfishes) is a speciose genus with over 100 species that occupy diverse habitats from the intertidal zone to deep (>1,000 m) water. Within this depth range, they are typically associated with high-relief rocky substrates; however, species display great variation in substrate affinity, ranging from cryptic reef-dwellers to semipelagic schooling species. The eastern North Pacific is the distributional center for Sebastes, with over 50 species occurring from California throug British Columbia (Eschmeyer and Herals, 1983). About 25 species occur in the western North Pacific (Masuda et al., 1984), four occur in the North Atlantic (Kendall, 1991) and at least one is found in the southern hemisphere off South America and Southe Africa (Chen, 1971).

Rockfishes are gonochoristic, with internal fertilization. Eggs incubate and embryos hatch in the ovaries, with subsequent extrusion of larvae. They have generally been regarded as classic examples of ovoviviparous fishes; however, recent studies have suggested that developing young use exogenous energy prior to birth (Boehlert and Yoklavich, 1984). Although females of many species of rockfishes release a single brood annually, some species produce multiple broods (MacGregor, 1970). Rockfishes are generally recognized as slow-growing (e.g., Archibald et al., 1981), with a suite of life history characteristics that places them in the K-selected group (Adams, 1980).

Reproductive parameters defining maturity schedules and fecundity are important life history characteristics. Cole (1954) explicitly identified mortality and reproductive variables as determinants of Darwinian fitness and established the paradigm that natural selection acts on a suite of life history characteristics to maximize fitness. Fishes have been used to analyze the accuracy of several life history models (e.g., Bell, 1980; Roff, 1984; Stearns and Koella, 1986). Fish-based data are useful because information on age at maturity, fecundity, and growth are available for a variety of species. The models all follow Cole (1954) in assuming that age at maturity, fecundity, growth, and mortality have coevolved to maximize Darwinian fitness. Observations that significant correlations exist among life history parameters (e.g., Adams, 1980; Pauly, 1980; Roff, 1981, 1984) suggest the assumption is correct.

Our summary of information describes maturity and fecundity as functions of length. Fecundity descriptions should be regarded with caution due to a number of inherent problems. Inconsistent methodology in determination of egg or embryo number may introduce considerable meaningless variation. Additional measurement variation is introduced when estimating fecundity of multiple spawning species (Grimes, 1987). In rockfishes, fecundity measurements also are a function of development stage, as fecundity decreases from pre-fertilization to the late embryo stage (Kusakari, In press). Consequently, fecundity estimates should be viewed as approximations, especially in those cases where we have extended the length-fecundity function outside the range of original observations (e.g., estimates of fecundity at maximum length).

The objectives of this review are to assemble the available information on maturity and fecundity in rockfishes, to explore the assembled data for patterns associated with geographic distribution and fish length, and to determine whether patterns are consistent with life history models.

Methods

Data describing length at maturity (length where 50 percent were mature), fecundity, growth, maximum reported length (MRL), and length-weight relationships were assembled from published literature and a limited number of unpublished sources. To identify sources we used our personal bibliographic reference systems, two recent bibliographic summaries (Clay and Kenchington, 1986; Leet and Reilly, 1988), and abstracting journals. Data from the northeast Pacific were assigned to geographic areas (Fig. 1.).

Some sources tabulated fecudity-length data for individual fish but provided no fit of data to a model (e.g., Phillips, 1964). In cases where the number of observations appeared adequate, we fit such data to the model F = a([L.sup.b]), using a least-squares algorithm. Lengths in the various sources were reported as standard, fork, or total length. We standardized all length measures to total length, based on the regression formulae provded by Echeverria and Lenarz (1985). For those species not included in that source, we used the mean regression coefficients for all species tabulated therein.

We used primary reproductive data (fecundity and length at maturity) and auxiliary data (growth described by the von Bertalanffy model, MRL, length-weight relationships) to generate derived reproductive data including fecundity at maturity (i.e., at length where 50 percent are mature), fecundity at MRL, fecundity per gram weight at maturity, and fecundity per gram weight at MRL.

The effect of latitude on maturity was examined by comparing the reported length at maturity between geographic areas, moving from area 1 northward through area 4 (Fig. 1). To determine whether females tended to mature at smaller or larger lengths than males, we compared the length at maturity for the two sexes in all species, including reports from all geographic areas. Differences were scored between areas or between sexes only if they differed by 2 cm or more. Results were tested for significance with chi-square tests, under the null hypotheses that comparisons scored as different would be evenly distributed between directions (north or south) or sexes.

The effects of fish size (length) on reproduction were examined through two linear regressions. In the first, length at maturity of females was the dependent variable and maximum reported length the independent variable. In the second, we regressed the ratio--fecundity at maturity: fecundity at maximum size--against size at maturity. We also plotted the trajectory of fecundity for each species from the length of 50 percent maturity of females to the maximum reported size for the species, using the fecundity-length parameters selected as typical for each species.

Results

Eastern North

Pacific Species

Data describing fecundity and maturity were collected for 45 rockfishes from 27 sources (Table 1). Estimates of length at maturity is more than one geographic area were available in 23 species. Fecundity estimates were usually limited to a single geographic zone. For analyses of reproductive characteristics among species, we used data that typified the fecundity and maturity for each species. If more than one source provided data on a particular parameter, we chose either the one from the geographic area nearest the center of distribution for that species, or the one with the most complete set of data. With few exceptions, data typifying a species are from a single area.

Length at maturity generally increased at the higher latitudes. Of 32 cases where female maturity could be compared between geographic areas, length at maturity in the northern area was greater in 20 cases, less in 4, and equal in 8 (P<0.05). In 30 cases where males were comparable, the length at maturity was greater in the north 21 times, less 5, and equal 4 (P<0.05). The only two species in which both males and females at matured at shorter length in a more northerly area were Sebastes jordani and S. levis, compared in zones 1 and 2 (Fig. 1).

Females tend to be larger at maturity than males, as this occurred in 29 cases whereas males matured at greater size only 5 times (P<0.01). In 42 cases there was no difference between the sexes. Female length at maturity varied from 9 cm (Sebastes dalli) to 52 cm (S. pinniger and S. ruberrimus). The mean length at maturity was 31 cm for females and 29 cm for males.

Length at maturity is related to maximum size attained by a species. Linear regression of female length at maturity on maximum length was highly significant (P<0.001, Fig. 2).

Fecundity as a function of length varied among species (Table 1). The exponent in the power equation ranged from 2.80 (Sebastes hopkinsi) to 5.51 (S. alutus), with a mean of 4.10. Calculated fecundity at maturity varied from 1,700 in S. dalli to 417,000 in S. paucispinis (Table 2), with a mean of 124,000. Calculated fecundity at maximum length varied from 35,000 for S. hopkins to 5.6 million for S. miniatus (Table 2), with a mean of 1.1 million.

Fecundity at maturity as a proportion of fecundity at maximum length varied from 0.01 to 0.25, with a mean of 0.09, and appears to be a positive function of size. A regression of that proportion on length at maturity was significant (p<0.05, Fig. 3).

We used fecundity per gram of body weight (FGB) as an indicator of relative investment in reproduction. At sexual maturity FGB ranged from 70 in Sebastes alutus to 325 in S. elongatus (Table 2), with a mean of 183. At maximum length

[TABULAR DATA OMITTED]

FGB ranged from 163 in S. alutus to 826 in S. miniatus, with a mean of 416. A regression of FGB on maximum length was not significant.

We plotted fecundity trajectories from fecundity at maturity to fecundity at maximum size (Fig. 4). The trajectories define an envelope of fecundity-at-length values for rockfishes. The species with data points outside the envelope are Sebastes alutus and S. marinus.

Western North

Pacific Species

We located data describing fecundity for six species of western North Pacific rockfishes: Sebastes inermis, S. pachycephalus, S. schlegeli, S. steindachneri, S. taczanowski, and S. vulpes (Table 3). Although we were unable to locate data sets suitable for assessing maturity (i.e., with indications of lengths of immature fish), the minimum length of females reported as mature in fecundity data allowed a very tentative indication of length at maturity, which ranged from 8

[TABULAR DATA OMITTED]

cm in S. pachycephalus to 34 cm in S. schlegeli.

Fecundity of western North Pacific rockfishes was low (Table 3), but typical of small species. When minimum and maximum sizes of fish in those studies were plotted with their associated fecundity estimates, the fecundity trajectories fell within the envelope defined by eastern North Pacific rockfishes. Furthermore, their lengths at maturity and fecundities are similar to those of eastern North Pacific species that mature at small sizes (e.g., Sebastes dalli, S. saxicola, S. semicinctus)

North Atlantic Species

In the western North Atlantic, there apparently are three Sebastes species, S. marinus, S. mentella, and S. fasciatus, although considerable taxonomic confusion has been associated with the latter two. Ni and Sandeman (1984) examined historical data on length at maturity in western North Atlantic populations of the three species. Length at maturity for S. marinus was 21-26 cm in males and 38-41 cm in females. The two other species were combined as the beaked redfishes, in which males matured at 16-29 cm and females at 24-43 cm.

Raitt and Hall (1967) reported length at maturity for female S. marinus from the eastern North Atlantic as 42 cm and 43 cm for populations near Iceland and the Faroe Islands, respectively. They also reported very similar fecundity estimates for those populations (included in Table 1). The fecundity trajectory from maturity to MRL is to the right of the envelope of fecundity trajectories based on eastern North Pacific species (Fig. 4).

Discussion

Increased maternal investment in progeny through viviparity or ovoviviparity is often assumed to result in decreased fecundity. Comparison of rockfish fecundity with oviparous fish taxa may indicate if total fecundity is indeed lower in

[TABULAR DATA OMITTED]

rockfishes as a result of their ovoviviparity. Grimes (1987) compiled fecundity estimates for snappers (Lutjanidae), a tropical family generally similar to rockfishes in size and shape. Mean FGB at maximum length for 13 snapper species was 731 (Grimes, 1987); in our compilation of 18 rockfish species, the mean FGB at maximum length was 416. His mean maximum fecundity for the 13 snapper species was 2.2 million at a mean maximum length of 55 cm. For rockfishes the mean maximum length was slightly lower at 52 cm; mean maximum fecundity was 1.1 million, half the value for snappers. However, there was considerable overlap in the ranges of fecundity parameters in rockfishes and snappers. Hislop (1984) summarized fecundity data for four gadoid species from the North Sea and estimated FGB for cod, Gadus morhua, and haddock, Melanogrammus aeglefinus, at 475 and 550, respectively. Those values are very similar to rockfishes, as are estimates of total fecundity at similar lengths. Rockfish fecundity apparently is lower than some comparable oviparous fishes, but the difference is not dramatic.

Increased length at maturity at higher latitudes is relatively common in eastern North Pacific rockfishes and has also been reported for rockfishes in the eastern North Atlantic (Ni and Sandeman, 1984). Increased size at maturity may be due to an extension in the juvenile period of northerly populations, for faster growth, or a combination of the two. However, few data are available for assessment of geographic variation in growth, and there is no indication of a general pattern. In the eastern North Pacific, Sebastes pinniger do not display geographic variation in growth, whereas S. diploproa grow faster in the north (Boehlert and Kappenman, 1980). Love (1978) also reported faster growth by S. serranoides to the north. Conversely, Westrheim and Harling (1975) reported a general trend of faster growth in southerly populations in seven out of eight rockfish species. Growth variation may be even more complex--Gunderson (1977) found male S. alutus grew faster in a southerly population, whereas females grew faster in the north. Given the lack of clear latitudinal trend in intraspecific growth, increased length at maturity in northern populations is likely due to delayed maturity.

There are several previous reports that female rockfishes mature at larger sizes than males of the same species. Ni and Sandeman (1984) found that females of Sebastes marinus, S. mentella and S. fasciatus all mature at larger sizes than males. Among 17 species of eastern North Pacific rockfishes, Wyllie Echeverria (1987) found that females matured at similar or larger sizes than males, and at older ages. Love et al. (1990) reported that in 7 out of 17 species, females matured at larger sizes than males.

Maturity at larger size in females could result from either later maturity or faster growth. Females have been reported to grow faster than males in Sebastes marinus (Kelly and Wolf, 1959), S. alutus (Westrheim, 1973), S. flavidus (Six and Horton, 1977, Fraidenburg 1980), S. melanops (Six and Horton, 1977), S. pinniger and S. diploproa (Boehlert and Kappenman, 1980), S. constellatus, S. elongatus, S. hopkinsi, S. ovalis, S. rosenblatti, S. saxicola, and S. semicinctus (Love et al., 1990). Although Wyllie Echeverria (1987) found older ages at maturity in females of several species, the widespread occurrence of faster growth in female rockfishes suggests that growth differences are a major factor contributing to larger size at maturity in females.

Among life history models, Bell's (1980) model appears generally consistent with a variety of life history patterns observed in fishes and other groups, and is based on an assumption that mortality rate decreases with increasing size. The model predicts that optimal size (or age) at maturity will occur when the rate of increasein fecundity equals the rate of decrease in survival. In other words, if juvenile mortality increases, relative to adult mortality, the optimum size of maturity increases; conversely, if adult mortality increases, relative to juveniles, the size at maturity should decrease. These patterns are consistent with the results of experimental studies (e.g., Barclay and Gregory, 1981), observations of a variety of wild populations (summarized in Stearns, 1977), and natural experiments comparing interpopulation variation of life histories in mosquitofish (Gambusia) in Hawaii (Stearns, 1983), guppies in Trinidad (Reznick, 1982) and shad in the eastern North Atlantic (Leggett and Carscadden 1978). Bell's (1980) model also leads to the prediction that females should mature later and/or at larger sizes than males; this was observed in Healey's (1986) analyses of salmon populations and is true for many rockfish species we reviewed.

If the relationship between adult and juvenile mortality determines size at maturity, the observed trend in maturity at smaller size in southern populations of rockfishes suggests that, relative to juveniles, adult mortality is higher in the south than in the north. This is not consistent with the suggestion by Boehlert and Kappenman (1980) that frequent episodes of low survival of larvae (due to offshore transport) has resulted in increased reproductive effort (and as a result, lower growth) in southern populations of S. diploproa. If southern populations experience increased larval mortality, Bell's model predicts that they should mature later, rather than earlier, relative to northern populations. Embiotocids, with no pelagic life stages, appear to be similar to rockfishes in having delayed maturity to the north (DeMartini et al., 1983). A possible mechanism might involve geographic differences in size-specific predation rates (Law, 1979).

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Lewis Haldorson is with the School of Fisheries and Ocean Sciencies, University of Alaska, Juneau, AK 99803, and Milton Love is with the Marine Science Institute, University of California, Santa Barbara, CA 93106.
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Publication:Marine Fisheries Review
Date:Mar 22, 1991
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