Growth backcalculations based on otoliths incorporating an age effect: adding an interaction term

An extension, including an age × length interaction, was developed for a recent model incorporating the age effect in the backcalculation of growth in length from otoliths. The interaction was demonstrated using empirical data from Arctic charr Salvelinus alpinus .     

Modelling seasonal increments in size to determine the onset of annual growth in fishes

A new method based on modelling of seasonal growth increments ( G _SI) in total length was found useful for assessing the date of onset of annual growth for 16 fish species in a temperate fluvial lake. Model comparisons indicated that polynomial (linear or quadratic) functions provided better fits to seasonal growth and were more likely to avoid convergence problems than alternative non-linear models. There was little evidence for differences in the date of onset of growth between years, nor among age classes within individual species. The onset of growth also was to some extent synchronized among species and was concentrated within a narrow temporal window of c. 2 weeks, between 18 May and 2 June, which corresponded to mean water temperatures between 16·1 and 17·3° C. There was no apparent relationship between date of onset and species' thermal preferenda or thermal preferences. By producing a point estimate along with appropriate 95% CI, the G _SI method provides useful information on the onset of growth and the uncertainty about that estimate. The G _SI analyses can contribute to a better understanding of environmental influences on the onset of growth and the length of the growing season, and of thermal thresholds for growth, including their use in calculation of degree-day metrics.     

Seasonal variability in growth of larval Japanese anchovy Engraulis japonicus driven by fluctuations in sea temperature in the Kii Channel, Japan

Seasonal variability in the growth of larval Japanese anchovy Engraulis japonicus was examined through otolith microstructure analysis based on the samples collected from the northern side (inner area, IA) and the southern side (outer area, OA) of the Kii Channel from April 2006 to March 2007. Growth trajectories (otolith backcalculated mean standard length of 5 day intervals from 5 days after hatch to 24 days) as well as the most recent 5 day mean growth rate of larvae before capture ( G ₅) differed among months. Growth trajectories showed the same pattern as G ₅. In IA, mean ± s.d. G ₅ ranged from 0·31 ± 0·04 mm day⁻¹ (January) to 0·73 ± 0·06 mm day⁻¹ (October). In OA, mean ± s.d. G ₅ ranged from 0·36 ± 0·05 mm day⁻¹ (January) to 0·79 ± 0·11 mm day⁻¹ (August). G ₅ values declined from November to January and then started to increase. In general, the seasonal patterns of growth were similar between IA and OA, and a clear seasonal pattern in growth was identified. When the relationships among larval growth rate, sea temperature, zooplankton density and larval density were examined, growth rate was positively related with sea temperature in both areas and not related with the other factors. The similar pattern in growth observed between IA and OA was probably due to the low spatial variability in sea temperature compared to its seasonal variability.     

Do benthivory and piscivory result in similar growth in walleye?

Growth and condition of piscivorous and non-piscivorous walleye Sander vitreus classified on the basis of their stomach contents were compared to assess the consequences of their trophic flexibility. Piscivorous males showed a significantly higher growth trajectory than non-piscivorous ones, while there was no difference in the growth of females. No differences were found in the condition of piscivorous and non-piscivorous walleye for either sex. Trophic specialization of males, with some individuals foraging mainly on invertebrates and others including forage fishes in their diets, seems to be the most plausible explanation for the observed growth differences. The results also suggest that invertebrates are an important component in walleye diet throughout its ontogeny.     

Relationship between scale growth checks, circuli formation rate and somatic growth in Rutilus rutilus (L.) a fish farm-reared cyprinid

Check formation on scales of roach Rutilus rutilus was examined during their production cycle at a fish farm in England through analysis of circuli patterns. Regular check formation was associated with the movement of fish from one type of grow-out facility to another; this resulted in a sudden shift in growth rate and the formation of a new check. As these had the characteristics of annual marks, their formation potentially invalidates their use as structures to determine the age of individuals during recapture events that may follow their introduction to the wild. At low growth rates, the number of circuli was constant throughout the year; this situation changed when fast growth rates were achieved. Five methods of backcalculation were also validated. When the proportionality between the body length and scale radii was weak, backcalculation methods were poor in determining length at check formation.     

BSE in France: epidemiological analysis and predictions

Attention throughout Europe continues to focus on bovine spongiform encephalopathy (BSE) with increasing evidence linking it to the new variant of Creutzfeldt-Jakob disease (vCJD) in humans. The age- and cohort-specific incidence of BSE in French cattle was modelled as a function of the survival distribution, the cohort-specific incidence of BSE infection, the underreporting rate of BSE cases, and the age-specific probability, conditional on survival, that an infected animal would experience clinical onset. The results reveal that thousands of French cattle were infected with BSE over the course of the epidemic. However, case incidence is predicted to decline in future years.     

Temporal patterns of growth in larval cohorts of the Japanese sardine Sardinops melanostictus in a coastal nursery area

Growth patterns of larval sardine Sardinops melanostictus were studied in a coastal nursery area, in southern Japan for four monthly hatch cohorts of larvae (November, December, January and February) for the 2003–2004 and 2004–2005 seasons. Laird–Gompertz models were fitted to each cohort using both total length (L_T)‐at‐age at capture and mean L_T‐at‐age data derived from backcalculations. In both approaches, the absolute daily growth rates (G_R) and absolute daily growth rates at the inflection point (G_XO) were estimated. In parallel, individual growth rates (G_I) were derived from backcalculated L_T (L_B). Growth showed the following general common patterns irrespective of hatch month, season and methods: (1) significant Laird–Gompertz fits, (2) asymptotic growth, (3) a decrease in G_R after the inflexion point, except for February for the 2003–2004 season that showed an apparent constant growth pattern, (4) six in eight cohorts showed G_XO ranging from 0·8 to 1·2 mm day^?1 and (5) a decreasing tendency of G_I from 1·75 to 0·24 mm day^?1, from first feeding through the first month of larval life. The contrasting pattern between the 2003–2004 and the 2004–2005 seasons were: (1) allometric v. logarithmic (ln) L_T and otolith radius relationships, (2) low G_XOv. high G_XO, (3) high G_Rv. low G_R when growth turned asymptotic, (4) low G_XOv. high G_XO when monthly hatch cohorts were combined and (5) L_B and G_I not differing among monthly hatch cohorts. The differences in growth patterns and growth rates between seasons seemed to be linked to the influx of warmer and oligotrophic waters of the Kuroshio Current that triggered an increase of 3° C in the coastal area for the 2003–2004 seasons. In the overall context, however, the high G_XO, within cohorts and seasons reported in the current study, suggests that either sea surface temperature (SST) or food availability, or both are in the optimal range of preferences for S. melanostictus larvae. Consequently, nearshore coastal areas seem to be playing an important role as a nursery area for the larval stage of this species.     

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 L ^2.99, 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 ( W_s ) was expressed by a power function of the W , W_s=0·731 W ^0·658. Carbon and nitrogen constituted 41·7 ± 1·5 and 10·0 ± 0·4% of the dry W_s , 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.     

The validity of different methods in the backcalculation of the lengths of roach—a comparison between scales and cleithra

The backcalculated lengths of roach Rutilus rutilus obtained with three different proportional methods [body proportional (BPH) and scale proportional hypotheses (SPH), Fraser—Lee] were compared. In the BPH and SPH methods both scales and cleithra were used, while the Fraser—Lee method was used with scales only. The validity of the results was evaluated by comparing the results with observed lengths within one cohort. Both body—scale and body-cleithrum relationships were best described by a power function ( r ²= 0·83). The backcalculated lengths of roach at age 1 varied from 51 mm (SPH, scales and cleithra) to 72 mm (Fraser—Lee), a 30% difference. In within-cohort comparisons, the difference compared with observed lengths was smallest in BPH (<4% with few exceptions). The linear Fraser—Lee method overestimated (up to 5%) the lengths at young ages and underestimated the lengths at older ages (3–5%). In SPH the difference with observed length was 12% at highest. The length estimates by SPH were more affected by the age—structure of the sample than the estimates by BPH, which suggested that Lee's phenomenon was present. This was tested by backcalculating the lengths at the age of 2 years for roach, separately from different age-groups. The backcalculated lengths declined with the increasing age of fish used in the calculations. The phenomenon was strongest in SPH and weakest in Fraser—Lee. The results suggested that in the backcalculation of the lengths of roach, BPH method gives the most reliable results. Considering the other methods, Fraser—Lee should be used rather than SPH.     

Age determination and backcalculation of pike length through use of the metapterygoid bone

The validity of the metapterygoid bone of pike Esox lucius for use in age determination and backcalculation of total length ( L _T) was evaluated by using tagged pike recaptured after 4 months to 3 years. A regression equation for the relationship between pike L _T and metapterygoid length was estimated, and no significant deviations between the backcalculated and expected L _T at tagging were found for recaptured pike. The results confirm that 1) the checks are formed annually and 2) no false checks appeared on the metapterygoids of pike.