Microsatellite analysis indicates an absence of population structure among Hippoglossus hippoglossus in the north‐west Atlantic

To explore the population structure of Atlantic halibut Hippoglossus hippoglossus , 160 fish from four locations in the north‐west Atlantic (Bay of Fundy, Scotian Shelf, Gulf of St Lawrence and Iceland) were examined for evidence of population structure using 18 microsatellite markers. Pair‐wise F _ST and a model‐based cluster analysis revealed no significant differentiation between samples, although uncertainties surrounding Atlantic halibut reproductive behaviour made it difficult to ascertain that only a single breeding population had been sampled at each location     

High resolution bomb dating for testing the accuracy of age interpretations for a short-lived pelagic fish, the Atlantic herring

Although stock assessments of Atlantic herring (Clupea harengus harengus) in the northwest Atlantic have relied on tens of thousands of annual age determinations each year for more than 20 years, recent analyses have suggested that there may have been systemic ageing error. Tracking of dominant year-classes and otolith exchanges confirmed the presence of substantial ageing bias among some readers, although these approaches could not be used to identify an accurate set of ages (if any). We applied bomb radiocarbon in a high resolution dating approach targeted at the 1962 year-class to assess ageing accuracy by multiple age readers and laboratories. Although bomb radiocarbon age validation studies are typically restricted to long-lived species, the availability of archived otoliths through the 1960s and 1970s made herring an ideal candidate for an approach targeted at a single year-class, and allowed the extent of any ageing error to be quantified. The results clearly demonstrated that current age reading practices under-aged fish >6 yr of age by up to 45%, although younger fish were aged accurately. Age underestimation was due to incorrect annulus interpretation rather than non-annual otolith growth. By focusing on the period of most rapid radiocarbon increase (1962), the margin of uncertainty around the targeted bomb radiocarbon ages was reduced to 0.66 yr. This study represents the first time the bomb dating method has reached sub-annual accuracy, which makes it well suited to the age validation of short lived fish species. The use of the targeted approach has considerable promise for improving the accuracy of other bomb radiocarbon studies without the problematic assumptions associated with curve estimation and environmental effects.     

Age validation and growth of three commercially important hemiramphids in south-eastern Australia

The method of using sectioned otoliths to estimate age in three species of garfishes (family Hemiramphidae) was validated by: (1) staining fishes with the vital stain alizarin complexone (ALC) and periodically examining their otolith growth, and (2) marginal increment analyses. Staining fishes with ALC indicated that opaque zones were formed during winter and spring, but did not become visible on the otolith edge until late spring and summer. Hyporhamphus australis were found to be similar to the hemiramphids of the Atlantic in having fast growth rates and a maximum observed age of 4+ years old. Hyporhamphus regularis ardelio and Arrhamphus sclerolepis krefftii were found to be more similar to the southern sea garfish, Hyporhamphus melanochir , in being moderately long-lived, with maximum observed ages of 7+ years old for both species. Females grew faster and attained greater fork lengths than males for each species. Sectioned otoliths showed large variation in the appearance of opaque zones between the three species studied, with those from the wide-ranging, oceanic H. australis appearing inconsistent and diffuse when compared to the estuarine H. r. ardelio and A. s . krefftii . This variation was also apparent from fishes kept in aquaria, suggesting that the appearance of opaque zones in otoliths of these species is largely influenced by physiology, rather than by environmental conditions.     

Preliminary estimates of the age and growth of immature smooth oreo Pseudocyttus maculatus Gilchrist 1906 (Oreosomatidae) in the Falkland Islands region of the South Atlantic

Sectioned otoliths were used to estimate the age of 29 specimens of the smooth oreo (Pseudocyttus maculatus Gilchrist 1906) from the Falkland Islands region of the South Atlantic. This represents the first ageing study of this species in this region of the world. All specimens were immature (sub-adults), the largest having a maximum total length of 347 mm. Growth increments were observed in sectioned otoliths and assumed to be annual in periodicity. The maximum age estimate of individuals in this study was 20 years and the form of somatic growth was similar to previous research on this species from Australian waters. The otolith microstructure consisted of an inner, mostly opaque zone with wide increments followed by an outer, mostly translucent zone with narrower increments. It is suggested that these zones correspond to the two growth phases in the life history of this species, namely the pelagic juvenile phase (opaque zone) and the demersal sub-adult/adult phase (translucent zone). Received: 27 May 1997 / Accepted: 31 October 1997     

Sexual dimorphism in the otolith shape of shi drum,Umbrina cirrosa (L.), in the eastern Mediterranean Sea: Fish size–otolith size relationships

Little is known about possible differences in sagitta otolith size and shape between sexes of the shi drum, Umbrina cirrosa, and relationships between their body and otolith size. Thus, this study aimed to fill this knowledge gap via examination of 414 sagittal otoliths from 108 male (total length 13.8–26.8 cm) and 99 female (13.5–26.7 cm) U. cirrosa caught between May 2017 and April 2018 in gillnets set at a depth of ~15 m in Mersin Bay, Eastern Mediterranean Sea. No statistical differences were observed between the shape indices of the left-sided and right-sided sagitta. However, there were significant differences in the size and shape of otoliths between males and females. The slopes of allometric power functions from otolith width × fish sizes gave significant differences between males and females (ANCOVA, P < 0.05). The relationship for length × weight of otoliths from both males and females showed isometric growth, whereas the relationship of otolith width × otolith weight showed positive allometry. Negative allometric growth was observed for the relationship otolith length × otolith width. In summary, this study revealed the presence of sexual dimorphism in the otolith shape of U. cirrosa, and the data on regression relationships of fish-otolith sizes can be used to estimate fish size from U. cirrosa otolith sizes.     

A nuclear DNA basis for shelf- and bank-scale population structure in northwest Atlantic cod (Gadus morhua): Labrador to Georges Bank

Variation at five microsatellite DNA loci scored in ≈ 1300 individuals provided evidence of genetic structure among 14 cod ( Gadus morhua ) populations spanning the range of the species in the northwest (NW) Atlantic. Using D _A and D _SW measures of genetic distance, as well as F _ST and R _ST measures of population structure, differences are revealed among populations at continental shelf scales (NE Newfoundland Shelf, Grand Banks, Flemish Cap, Scotian Shelf, Georges Bank) where regions are separated by submarine saddles, channels and trenches. However, we also provide evidence of genetic structure at spawning-bank scales consistent with variation in oceanographic features and in the spatiotemporal distribution of spawning, each of which may represent barriers to gene flow among geographically contiguous populations inhabiting a highly advective environment. The differences described are consistent with postdispersal spawning fidelity to natal areas, a behaviour that may be facilitated by topographically induced gyre-like circulations that can act as retention mechanisms. Significant degrees of substructure among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conceivably form the template for the evolution of the structure. We suggest that bathymetric and hydrodynamic structure represents a rational starting point for developing hypotheses to examine the processes that lead to the genetic structuring of marine fish species.     

New information on age and growth in length of Micromesistius australis,Norman 1937 (Pisces,Gadidae), in the South-West Atlantic

Summary Sagittal otoliths were used for age determination in 1121 specimens of Micromesistius australis, caught in waters of the South-West Atlantic during the cruises made by the ships Walther Herwig and Shinkai Maru in agreement with the Argentine Republic, during the years 1978 and 1978/1979, respectively. The interpretation of growth rings required the otoliths to undergo a paraffin wax embedding and cutting technique. The relations otolith length-fish length and fish length-half otolith width were linear. Thus, back calculations, when necessary, were made by a simple extrapolation. Numbers of males and females were nearly equal, resulting in a sex ratio of 1:1.09. Length frequencies and age distributions were calculated for each sex separately. A significant difference in growth between sexes was found, curves for each were derived. A linear, regression was made and the Von Bertalanffy equation estimated as: Lt=56.90 [1-e^{-0.1983 (t+1.84)}], in males, and Lt=60.08 [1-e^{-0.1769 (t+2.07)}] in females, with Lt in cm and t in years. For estimation of these equation parameters age groups 2–16 years in males and 2–22 years in females were taken into account. All theoretical Lt values and almost all observed lengths for females were greater than those of males in corresponding age groups. Previously published observations about M. australis on this subject, are not in agreement or only in partial accord with present results. These are discussed. Original photographs of otolith cross sections illustrating age determination methods are presented for the first time in age and growth studies of this species.     

Temperature effects on otolith pattern formation in Atlantic cod Gadus morhua

The effect of food intake and temperature on otolith macrostructure and microstructure was examined experimentally in Atlantic cod Gadus morhua. Daily increment formation was validated and otolith accretion rate and optical density quantified using image analysis. Two‐week periods of starvation had no discernable effect on otolith increment width or optical density, despite having negative effects on somatic growth. In contrast, temperature had a strong positive effect on otolith accretion rate and clear effects on optical density with the otolith becoming more translucent at higher temperatures. Somatic growth, otolith accretion and otolith optical density each had a significantly different response curve to temperature. No relationship was detected between individual somatic growth rates and the accretion rate or optical properties of the otolith. The experimental manipulation of temperature‐induced otolith patterns similar to the ‘false ring’ secondary structures sometimes observed in the otoliths of wild fish. The results suggest that otolith pattern arises from a combination of temperature and seasonal effects, but not directly from individual variation in somatic growth.     

Otoliths speak out: why the Pacific halibut in Puget sound are different

Pacific halibut, Hippoglossus stenolepis, is one of the most important commercial groundfish and is managed as a single coast-wide population from Alaska to northern California. Nevertheless, genetic investigations did not show success in detecting the population structure of the species. Here I report stable oxygen and carbon isotope analyses (δ¹⁸O and δ¹³C) in otoliths to discriminate the stock differences from two sample locations between the Washington coast (WC) and the northern Puget Sound (PS), and two sample years in 2007 and 2008. In general the δ¹⁸O values of halibut otoliths from WC ranged from ?0.2 to 1.8‰, higher than the PS samples from ?0.5 to 1.4‰. In contrast, the δ¹³C values from WC ranged from ?3.6 to ?1.0‰, lower than the PS samples from ?3.2 to ?1.2‰. Results from the otolith nuclei (age-0 halibut) and the 8^th (the earliest maturity age for male halibut) and edge otolith rings (the latest location where the fish lived) showed significant differences between halibut samples from PS and WC. In particular, the sample location difference (between PS and WC) in both δ¹³C and δ¹⁸O data was significant and markedly larger than the sample year difference (between 2007 and 2008). These isotopic signatures provide evidence that the PS halibut may belong to a distinct stock that is significantly different from WC halibut.     

Age and growth of the dusky grouper Epinephelus marginatus (Lowe 1834) in an exploited population of the western Mediterranean Sea

The age and growth of the dusky grouper, Epinephelus marginatus , in the Balearic Islands (western Mediterranean) were studied by otolith analysis from a sample of 358 specimens ranging in total length ( L _T) from 6·6 to 105·6 cm. The specimens came from commercial artisanal and recreational spear fisheries between 1999 and 2003. Otoliths grew asymmetrically throughout the range of L _T studied, showing a clear pattern of alternating translucent and opaque bands. Marginal increment analysis of specimens up to 8 years-old indicated that a single opaque band was formed each year during spring and summer. Whole otoliths allowed ageing specimens up to 10 years old, but above that age whole otoliths yielded lower age estimates than sectioned otoliths. The maximum estimated age was 61 years, which significantly extends the estimated life span of the species from a maximum of 36 years in a previous study. The von Bertalanffy growth parameters were estimated as L _∞= 95·5 cm L _T, K = 0·087 and T _O=−1·12. The study revealed differences in mean L _T at age and age structure between the shallow- and deep-water samples which may be attributed to different fishing pressure and environmental conditions.     

Do otolith increments allow correct inferences about age and growth of coral reef fishes?

Otolith increment structure is widely used to estimate age and growth of marine fishes. Here, I test the accuracy of the long-term otolith increment analysis of the lemon damselfish Pomacentrus moluccensis to describe age and growth characteristics. I compare the number of putative annual otolith increments (as a proxy for actual age) and widths of these increments (as proxies for somatic growth) with actual tagged fish-length data, based on a 6-year dataset, the longest time course for a coral reef fish. Estimated age from otoliths corresponded closely with actual age in all cases, confirming annual increment formation. However, otolith increment widths were poor proxies for actual growth in length [linear regression r ² = 0.44–0.90, n = 6 fish] and were clearly of limited value in estimating annual growth. Up to 60 % of the annual growth variation was missed using otolith increments, suggesting the long-term back calculations of otolith growth characteristics of reef fish populations should be interpreted with caution.     

The detection of elements in larval otoliths from Atlantic herring using laser ablation ICP-MS

Trace element concentrations of otoliths from larval herring Clupea harengus collected from known spawning beds in the Celtic and Irish Seas, were investigated using laser ablation ICP-MS and compared with concentrations in the larval cores of juvenile otoliths from the same populations and year class. A range of elements (Mg, Zn, Sr, Ba and Pb) was detectable in early larval otoliths (20–40 µm diameter). Larval otolith concentrations exceeded the larval core concentrations of juvenile otoliths and also the concentrations reported in the literature, for Mg, Zn, Ba and Pb, indicating that the measurement of elements in larval otoliths was severely affected by post-mortem contamination, most likely due to adherence of tissue and endolymph residue on the otolith surface. Comparison of otolith composition between larvae from two freezing treatments showed that contamination from Mg and Zn was more serious in otoliths that had remained in frozen larvae for prolonged periods. Larval populations from the two seas showed significant differences in otolith Sr concentrations, which were consistent over two sampling years. Similar differences were seen in the corresponding juvenile populations. The results show that while early larval otoliths are extremely susceptible post-mortem contamination, Sr concentrations can be reliably measured using laser ablation ICP-MS and for this element, the detection of region specific differences is possible.     

Is otolith science transformative? New views on fish migration

Publications focused on fish otolith applications grew exponentially from 1981–2000 but have subsequently stabilized, suggesting maturation of otolith applications. Over the past 30 years >3,500 primary journal publications were identified prompting the question, how have otoliths fundamentally changed our understanding of fishes and their environment? Has otolith science in this way been transformative? I use Harden Jones’ 1968 Fish Migration as a benchmark of fish migration concepts prior to Pannella’s 1971 breakthrough paper on otolith microstructure. For case study species I highlight how otolith science has informed migration concepts including, (1) parent stream theory (Atlantic bluefin tuna), (2) adoptive homing (Atlantic herring), and (3) partial migration (European eel). Harden Jones’ overall conclusion that life cycle closure leads to population structure (also known as the migration triangle) is a first principle in fisheries science, but in recent years has been challenged by otolith science. In particular, a transformative discovery attributable to otoliths is that life cycles vary substantially within populations. New avenues of otolith science are now exploring the causes and consequences of this life cycle diversity, and large advances are expected through integration of otolith approaches with electronic tagging, genetics, field manipulations, and modeling that will permit migration concepts to be tested at multiple ecological scales.     

Estimating the timing of growth rings in Atlantic cod otoliths using stable oxygen isotopes

A technique involving micro‐scale sampling of otolith carbonate and analyses of stable oxygen isotope composition was used to relate the zone appearance of the otolith to the seasonal temperature cycle. Otolith opacity could then be related to the timing of zone formation. Otoliths from two groups of Atlantic cod Gadus morhua held under known temperature conditions over a period of 4 and 6 years were examined. The otolith translucency followed the same pattern as the estimated temperature (from otolith δ¹⁸O values) in the yearly increments three and four, meaning that the translucent zones were deposited at the seasonal highest temperature in late summer and early autumn. The relative light intensities of otolith yearly increments five and six of older fish (deposited in the same years), however, were not significantly correlated to the estimated temperatures since increased otolith translucency also occurred at low temperatures. This might have been caused by stress in connection with gonad development or starvation during the spawning period. The results showed that this method of coupling otolith opacity and stable oxygen isotope composition can be used to estimate the timing of zone formations in otoliths.     

The effect of growth rate,size, and season on oocyte development and maturity of Atlantic cod (Gadus morhua L.)

The effect of growth rate, body weight, age, and season on ovarian development and maturation was investigated for Atlantic cod (Gadus morhua L.) reared in the laboratory over 10 months, for each of two consecutive years, 1978–1980. Cod were also collected from the Gulf of St. Lawrence, the Scotian Shelf, Georges Bank, the Flemish Cap, and the N.E. Newfoundland Shelf.The state of maturity was recognized by oocyte size. Stage I oocytes did not vary in size with growth rate, season, age, or maturity, while the size of stage II oocytes was positively correlated with maturity and negatively correlated with maximum stage of development achieved. Ovarian wall thickness was positively correlated with age and maturity.The frequency distribution of stage I and II oocytes distinguished the state of maturation, with cod that would mature by the next spawning season having a minimum of 20% ($\text{x}\text{?}\text{= 42\%}$) of their oocytes at stage II or greater level of development.Maturing 3-yr-old cod had greater life specific growth rates than immature 3-yr-olds, but growth rates during the third year itself were not significantly different. A hypothesis of a three-part density-dependent mechanism controlling fecundity is postulated. Future reductions in partial recruitment and total fecundity are predicted for the Gulf of St. Lawrence cod stock based on calculated growth rates for Gulf cod in 1979.     

Microsatellite variation in cod Gadus morhua throughout its geographic range

Previous genetic studies using neutral markers such as allozymes, mtDNA and minisatellite loci have demonstrated varying amounts of population structure in cod Gadus morhua throughout the Atlantic. Microsatellite loci, which are potentially the most informative of presently available neutral genetic markers, have been applied extensively within western and eastern Atlantic areas but not on a range-wide basis. In the present study, six microsatellite DNA loci were used to screen cod samples from nine locations throughout the geographic range from the Scotian Shelf in the West Atlantic to the Barents and Baltic Seas in the east. Overall F _ST value was 0·03 ( P = < 0·001) across all samples. Statistically significant population differences over all loci combined were evident between more geographically distant samples, using either heterogeneity tests or F _ST analysis, with at least one locus showing significant differences between all samples (prior to Bonferroni correction). A significant correlation was observed between genetic and geographical distance, suggesting a higher level of historical and contemporary gene flow between adjacent populations than more distant populations. Samples from either end of the geographic range (Scotian Shelf and Baltic Sea) were particularly distinct when analysed using the STRUCTURE programme and also showed a high level of self-assignment when individuals of either the Scotian Shelf or Baltic Sea were tested against the entire data set. The present microsatellite study demonstrates a high level of geographic population structure between the western Atlantic, middle and eastern Atlantic and Baltic Sea, and thus, the findings should be useful in devising overall management and conservation strategies for the species.     

Growth pattern of the young of the year Argentine hake <Emphasis Type="Italic">Merluccius hubbsi</Emphasis> Marini, 1933 (Gadiformes Merluccidae) along the Brazilian and Uruguayan coasts

Morphology and morphometry of the sagittae otolith were studied in young of the year Argentine hake, Merluccius hubbsi in the Southeastern Atlantic. Geographical variation in the growth pattern of the young of the year Merluccius hubbsi was correlated with the differences found in otolith morphometry, relative growth, ring positions, as well as the formation of a new ring. The otolith development of M. hubbsi throughout its area of distribution accompanies an increasing northward temperature gradient, resulting in bigger otoliths and greater variability of ring position from northern to southern area. The canonical discriminate analysis showed that the otolith length and height explained most of the variation of the first discriminant function. We found significant differences in the discriminant scores between those samples from northern and southern area. As regards the Uruguayan coast and Southern Brazil, age groups zero shared some similarity as well as differences in growth; environmental features explain these differences but their effect on the adult fishes growth is still unknown.     

The use of otolith microstructure to estimate age in adult Atlantic cod Gadus morhua

Transverse sections of otoliths from Atlantic cod Gadus morhua from the Baltic Sea revealed narrow growth increments. The widths of these increments corresponded to daily increments from fish with known otolith growth rates and were therefore assumed to be daily increments. They exhibited a distinct pattern with increasing distance from the primary primordium. A series of zones with clearly distinguishable increments, first with increasing then with decreasing widths in a dome‐shaped pattern, were separated by zones where no regular increment structure was visible. Increment width seemed to be tightly coupled to the annual cycle in environmental temperature at a depth of 30–60 m, where G. morhua predominantly reside. Between 135 and 200 increments occurred within the different zones, with a non‐significant trend towards lower increment numbers and widths with distance from the primary primordium of the otolith. Increment formation apparently ceased at temperatures < 5–6° C, but growth during the cold months corresponded closely with estimated growth rates. The increment patterns seemed to reflect annual cycles in environmental temperature, and the count of the increment cycles may thus be a promising tool for the determination of the true age of Baltic G. morhua.