Biological Intercept Research Articles

An otolith-based back-calculation method to account for time-varying growth rate in rainbow smelt (Osmerus mordax) larvae

This study develops a new back-calculation method, based on the larvae of rainbow smelt (Osmerus mordax), that takes into account the variation of growth rate over time. Known-aged larvae were reared in four 60-L microcosms during 49 days in order to obtain a large range of individual growth trajectories. We first validated the daily nature of the otolith increment deposition rate. The proposed time-varying growth (TVG) method weights the contribution of each increment in the length calculation using a growth effect factor. A small growth increment contributes less to the length increase of larvae than its relative importance in total otolith growth. On the other hand, a large growth increment contributes more to the length increase of larvae in comparison with its relative importance in total otolith growth. This method provided significantly better estimates of previous length-at-age than the biological intercept (BI) method at the individual level. In addition, the TVG method tended to provide more accurate estimates of previous length-at-age than the BI method at the population level, but the difference was not significant. The importance of using the TVG method instead of the BI method to back-calculate individual and population growth trajectories increases with the magnitude of the growth effect and the variation in growth rates over time.

Feeding and growth of early juvenile Japanese sardines in the Pacific waters off central Japan

Larval and early juvenile growth was backcalculated for individual Japanese sardines Sardinops melanostictus using the biological intercept method based on the allometric relationship between otolith radii and fish lengths. Sardines grew at 0·81 mm day−1 during the larval stage. In the early juvenile stage, they grew from 32·3 to 45·4 mm fork length (L) over a 20-day period (0·64mm day−1). Using the observed relationship between L and wet body weight (W), W = 0·00942 L2.99, W of the sardine juveniles was calculated to increase from 306 to 832 mg during the 20-day period. The carbon (C) requirement to achieve this growth in weight was estimated to increase from 5·7 to 9·6 mg day−1. Stomach contents of the sardines were composed mostly of copepods (73%) and larvaceans (25%). Wet stomach content weight (Ws) was expressed by a power function of the W, Ws=0·731 W0·658. Carbon and nitrogen constituted 41·7 ± 1·5 and 10·0 ± 0·4% of the dry Ws, respectively. Stomach C content increased from 2·0 to 3·9 mg during the 20-day period. Three to four cycles of the daily turnover of stomach contents during the 16 h of daytime, corresponding to a gastric evacuation rate of 0·2–0·3 h−1 under continuous feeding, met the C requirement to achieve the backcalculated growth in early juvenile sardines. The Kuroshio frontal waters seem to provide Japanese sardine juveniles with favourable growth conditions.

Feeding and growth of early juvenile Japanese sardines in the Pacific waters off central Japan

Larval and early juvenile growth was backcalculated for individual Japanese sardines Sardinops melanostictus using the biological intercept method based on the allometric relationship between otolith radii and fish lengths. Sardines grew at 0·81 mm day−1 during the larval stage. In the early juvenile stage, they grew from 32·3 to 45·4 mm fork length (L) over a 20‐day period (0·64mm day−1). Using the observed relationship between L and wet body weight (W), W = 0·00942 L2.99, W of the sardine juveniles was calculated to increase from 306 to 832 mg during the 20‐day period. The carbon (C) requirement to achieve this growth in weight was estimated to increase from 5·7 to 9·6 mg day−1. Stomach contents of the sardines were composed mostly of copepods (73%) and larvaceans (25%). Wet stomach content weight (Ws) was expressed by a power function of the W, Ws=0·731 W0·658. Carbon and nitrogen constituted 41·7 ± 1·5 and 10·0 ± 0·4% of the dry Ws, respectively. Stomach C content increased from 2·0 to 3·9 mg during the 20‐day period. Three to four cycles of the daily turnover of stomach contents during the 16 h of daytime, corresponding to a gastric evacuation rate of 0·2–0·3 h−1 under continuous feeding, met the C requirement to achieve the backcalculated growth in early juvenile sardines. The Kuroshio frontal waters seem to provide Japanese sardine juveniles with favourable growth conditions.

Size-Selective Mortality of Inland Silversides: Evidence from Otolith Microstructure

Size-selective mortality of age-0 inland silversides Menidia beryllina was assessed by using otolith microstructure analysis and the biological intercept back-calculation procedure. Back-calculated size-frequency distributions were compared with the size-frequency distributions of field collections to determine if and when size-selective mortality occurred. These analyses indicated that in both 1990 and 1992, size-selective mortality occurred and was directed at the larger members of the inland silverside cohort. The timing of size-selective mortality varied between the 2 years. In 1990, size-selective mortality occurred during the juvenile stage, whereas in 1992, size-selective mortality occurred during the late larval and early juvenile stages. These results conflict with the prevailing hypothesis that larger individuals have higher survival probabilities than smaller individuals but support our previous finding that juvenile bluefish Pontatoinus saltatrix prefer to prey on the larger members of the inland silverside cohort.

Temporal variation in the early life-history characteristics of the King George whiting (Sillaginodes punctata) from analysis of otolith microstructure

This study describes the duration of the settlement season, the somatic and otolith growth rates, and presettlement durations for Sillaginodes punctata at Barker Inlet, South Australia. The settlement season was from June to November, with settlement occurring in two phases over this period. Somatic growth rates ranged from <0.1 to 0.25 mm day-1 depending on age and time of year, making size (SL) a relatively poor indicator of age. Alternatively, otolith size (OL) was strongly related to age, but the linear relationships varied systematically among sampling occasions. Because of variation in somatic growth rates, the SL-OL relationships were relatively poor. The biological intercept method was used to back-calculate fish sizes from otolith increment widths for three samples of fish. These growth trajectories differed considerably, two being logistic in shape and the third being an exponential relationship. Presettlement durations increased from 80 to 130 days between June and September and were inversely related to growth rate. Settlement competence is related more to size than to age. The broad natural variation in early life-history characteristics is likely to relate to water temperature regimes along larval advection pathways through the long settlement season.

Comparison of Otolith-Based Back-Calculation Methods to Determine Individual Growth Histories of Larval Striped Bass, Morone saxatilis

In rearing studies on 6- to 22-d-old larval striped bass, Morone saxatilis, we applied several back-calculation methods to known-growth larvae. A growth effect occurred on otolith diameter – standard length relationships, where slower growing larvae had relatively larger otoliths. Otolith growth was less affected by feeding regime than was somatic growth. Due to the conservative nature of otolith growth, proportional based (Biological Intercept Method) and simple linear regression methods linearized somatic growth transitions and did not estimate periods of negative growth. A quadratic regression method which used age as an additional predictor resulted in the accurate back-calculation of size at age in all groups of laboratory-reared larvae. However, when model coefficients were applied to a test population of pond-reared larvae, the quadratic model performed poorly. While differences in relative otolith size between pond- and laboratory-reared larvae could be ascribed to a temperature effect, the inability to apply the model also indicates a problem specific to regression-based methods. Theoretical rationale and experimental proof provided evidence for the inclusion of age in back-calculation models, but parameterization will have to occur for each field application.

How Reliable are Growth Back-Calculations Based on Otoliths?

Growth back-calculations from otoliths assume that the relationship between fish and otolith length is linear through time. The final (or observed) individual fish-otolith ratios are then combined to prepare a fish-otolith regression for the population, upon which the subsequent back-calculations are based. However, recent studies have demonstrated that the fish:otolith size ratio varies systematically with somatic growth rate, resulting in relatively large otoliths in slow-growing fish. Such a growth effect will result in a fitted fish-otolith regression which differs significantly from that of the mean of the individual fish-otolith slopes. Fraser–Lee growth back-calculations made from such a regression consistently underestimate previous lengths at age. The bias may explain the apparent ubiquity of Lee's phenomenon. Back-calculation bias was eliminated through use of an algorithm defining individual fish-otolith trajectories and a biologically determined, rather than a statistically estimated, intercept. Adaptations of the biological intercept back-calculations procedure accurately predicted previous lengths in the presence of both stochastic error and time-varying growth rates. When used to reevaluate some published back-calculations, the biological intercept procedure resulted in more accurate values than those previously estimated, and reduced or eliminated the presence of Lee's phenomenon.