Fish otoliths: do sizes correlate with taxonomic group,habitat and/or luminescence?

Otoliths are dense structures in the ears of fishes that function in hearing and gravity perception. Otolith (sagitta) diameters, as percentages of standard length (% SL), are calculated for 247 marine fish species in 147 families and compared by taxonomic group (usually order), habitat and presence or absence of luminescence. Otolith sizes range from 0.4-31.4 mm and 0.08-11.2% SL. The eel and spiny eel orders Anguilliformes and Notacanthiformes have small to very small otoliths, as do the triggerfish order Tetraodontiformes, pipefish order Gasterosteiformes, billfish suborder Scombroidei and many of the dragonfish order Stomiiformes. The soldierfish order Beryciformes has moderate to very large otoliths. The perch order Perciformes has a wide range of otolith sizes but most have small to moderate otoliths 2-5% SL. Only 16 out of the 247 species have the relatively largest otoliths, over 7% SL. Seven out of these 16 species are also luminous from a variety of habitats. Luminous species have slightly to much larger otoliths than non-luminous species in the same family Both beryciforms and luminous fishes live in low-light environments, where acute colour vision is probably impossible. Most fishes of the epipelagic surface waters have very small otoliths, perhaps due to background noise and/or excessive movement of heavy otoliths in rough seas. Bathypelagic species usually have small otoliths and regressed or absent swimbladders. Other habitats have species with a range of otolith sizes. While the relationship between hearing ability and otolith length is unknown, at least some groups with modified swim-bladders have larger otoliths, which may be associated with more acute hearing.     

Frequency of vateritic otoliths and potential consequences for marine survival in hatchery-reared Atlantic salmon

Otoliths are inner-ear structures of all teleost fish with functional importance for hearing and balance. The otoliths usually consist of aragonite, a polymorph of calcium carbonate, but may also take the form partly or entirely of vaterite, a different polymorph of calcium carbonate. Vateritic otoliths occur sporadically in wild fish, but with a higher frequency in hatchery-reared fish. Abnormal otoliths have direct consequences for the inner-ear functions of fish and may be a symptom of environmental stress. In this study, the authors assess the differences in the frequency of abnormal otoliths and degree of abnormality (% vaterite) for different groups of hatchery-reared Atlantic salmon (Salmo salar) smolt and adults. The groups differed in parental broodstock origin (number of generations in hatchery) and treatment temperature. Smolt from the same groups were also released to complete their ocean migration. The otoliths of the returning and recaptured adults were subsequently extracted to assess the difference in frequency and degree of abnormality between the adults and the smolt from corresponding groups. Return rate varied among groups (0.2%–2.6%). The frequency of vateritic otoliths was high (11.4%–64.4%) and differed among smolt groups. The lowest return rates corresponded with the highest frequency of abnormal otoliths for the groups, suggesting that abnormal otoliths may have negative consequences for marine survival. Furthermore, indications of an effect of fast growth on the formation of abnormal otoliths were found for only one of the experimental groups, and for none of the groups after correcting for Type 1 error. This contradicts previous reports, suggesting rapid growth as the main cause of abnormal otoliths. Adult return rates were generally low, but abnormal otoliths were common, with high coverage (% vaterite).     

Using the otolith sulcus to aid in prey identification and improve estimates of prey size in diet studies of a piscivorous predator

Diet studies are fundamental for understanding trophic connections in marine ecosystems. In the southeastern US, the common bottlenose dolphin Tursiops truncatus is the predominant marine mammal in coastal waters, but its role as a top predator has received little attention. Diet studies of piscivorous predators, like bottlenose dolphins, start with assessing prey otoliths recovered from stomachs or feces, but digestive erosion hampers species identification and underestimates fish weight (FW). To compensate, FW is often estimated from the least affected otoliths and scaled to other otoliths, which also introduces bias. The sulcus, an otolith surface feature, has a species‐specific shape of its ostium and caudal extents, which is within the otolith edge for some species. We explored whether the sulcus could improve species identification and estimation of prey size using a case study of four sciaenid species targeted by fisheries and bottlenose dolphins in North Carolina. Methods were assessed first on otoliths from a reference collection (n = 421) and applied to prey otoliths (n = 5,308) recovered from 120 stomachs of dead stranded dolphins. We demonstrated in reference‐collection otoliths that cauda to sulcus length (CL:SL) could discriminate between spotted seatrout (Cynoscion nebulosus) and weakfish (Cynoscion regalis) (classification accuracy = 0.98). This method confirmed for the first time predation of spotted seatrout by bottlenose dolphins in North Carolina. Using predictive models developed from reference‐collection otoliths, we provided evidence that digestion affects otolith length more than sulcus or cauda length, making the latter better predictors. Lastly, we explored scenarios of calculating total consumed biomass across degrees of digestion. A suggested approach was for the least digested otoliths to be scaled to other otoliths iteratively from within the same stomach, month, or season as samples allow. Using the otolith sulcus helped overcome challenges of species identification and fish size estimation, indicating their potential use in other diet studies.     

Fish prey of Antarctic fur seals,Arctocephalus gazella,during the summer-autumn period at Laurie Island,South Orkney Islands

The fish component in the diet of Antarctic fur seals, Arctocephalus gazella, was investigated at Laurie Island, South Orkney Islands from mid January to April 1988. Fish otoliths occurred in 78.8% of faecal droppings. Most of the otoliths extracted from scats belonged to Myctophid fish (93.4%), mainly Electrona antarctica and Gymnoscopelus nicholsi. These two fish species thus constituted the bulk of the diet. A greater percentage of the otoliths from G. nicholsi (31.1%) in this study were considered suitable for measurement in comparison with 10.3% from scats at Heard Island in 1990 and 11.4% at Macquarie Island in 1988/89. This lesser degree of erosion would suggest that foraging areas of fur seals during this study were closer to land than during previous studies. During the period studied, the commercial fishery around the South Orkneys was not based on Myctophid fish so there was no direct competition for fish between the fur seals and fisheries.     

Assessment of errors associated with harbour seal (Phoca vitulina) faecal sampling

Six harbour seals, ages 4–8 years, were held as pairs in a 10 times 20 times 2 m tank filled with sea water, and on 60 occasions were fed a meal of a specific species of fish or cephalopod of known size. The tank was drained periodically, and harbour seal faeces were collected on a 0.5 mm sieve. Number and size of otoliths and beaks found in faeces were determined. Fifty-eight percent of 670 fish and 37% of 36 cephalopods fed to harbour seals were represented by their otoliths or beaks in faeces. Estimated number of prey consumed was determined from the greatest number of left or right otoliths or upper or lower beaks collected in faeces. Estimated length ofprey was determined from measurements of otoliths and beaks recovered in the tank and relationships of otolith and beak measurements to prey length. Estimated number of fish eaten was not significantly different among pairs of harbour seals, but was different among species of fishes. Only 24–35% of fish species with small otoliths were represented in faeces, whereas more robust otoliths from other species were less apt to be completely dissolved. Estimated length of fishes was significantly less than lengths of fishes fed to harbour seals in 39 (76.5%) of 51 trials. Cephalopod beaks were not affected by passage through the harbour seal digestive tract. Amount of otolith dissolution was not related to species of fish; estimated fish length was underestimated by an average 27.5%. Although some (7.4%) of the otoliths were collected within 100 h after the fish were ingested, more than 90% were recovered within 24 h after the fish was eaten. Correction factors were developed which will allow researchers to estimate more reliably number and size of fish and cephalopod prey eaten by harbour seals.     

A comparison of different structures and methods for estimating age of northern-form Dolly Varden <Emphasis Type="Italic">Salvelinus malma malma</Emphasis> from the Canadian Arctic

Assessment of anadromous northern-form Dolly Varden Salvelinus malma malma in the Western Canadian Arctic requires reliable methods for estimating ages. Additionally, conservation efforts warrant determining whether fin rays provide a non-lethal alternative to otoliths. Precision and bias of whole and sectioned otoliths, and sectioned pectoral and pelvic fin rays were examined. Two age readers with different levels of experience ageing this species read each structure three times. Coefficient of variation (CV) was calculated to measure precision, and age bias plots were created for each method of preparation/structure within and between readers. The experienced reader demonstrated the highest precision with sectioned otoliths (CV = 1.6 %) followed by whole otoliths (CV = 4.2 %) while pectoral and pelvic fins were the lowest, CV = 7.7 % and 7.5 %, respectively. The age bias plot showed little difference between whole and sectioned otoliths, although greater imprecision/bias was evident for whole otoliths at age ≥9. Compared to otoliths, fin rays produced younger age estimates starting at 5 years; however, pelvic fins were more biased towards younger estimates than pectoral fins. The less experienced reader had greater inconsistencies, tending to overage younger and underage older samples for all methods compared to the more experienced reader, underscoring the importance of experience when estimating age for this species. We conclude that both types of fin rays are a poor non-lethal alternative to otoliths for fish ≥5 years and recommend an experienced ager could use whole otoliths up to age 8 and sectioned otoliths for fish ≥9 years (>500 mm fork length).     

Morphology and microchemistry of abnormal otoliths in the ayu, <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>

The sagittal otolith morphology and microchemistry of reared juvenile ayu, Plecoglossus altivelis, were examined to describe the occurrence and microchemical characteristics of the abnormal otoliths in this species. Juvenile ayu (N = 31) were collected in June 2004 at three different locations, Wakayama, Kumamoto, and Biwa Lake in Japan, where they were being reared in freshwater aquaculture ponds after having been collected in the wild as larvae. Otolith abnormality was found in the sagittae of 26% (N = 8) of the individuals examined, of which five fish had abnormal otoliths only on one side, while the otolith on the other side was normal. Abnormal otoliths were more transparent and crystalline in appearance with irregular shapes compared to normal ones that were more opaque and less irregular. Abnormal otoliths were divided into two types, semi-abnormal (Type 1) with a normal part in the center, and fully-abnormal (Type 2) that were completely crystalline in appearance. The line transects and whole otolith concentration maps showed that the contents of Sr, Na and K were lower in the abnormal otolith regions compared to the normal ones, while those of Ca and S were almost constant in both. The appearance and microchemical properties of the abnormal ayu otoliths were similar to the abnormal otoliths in other species in which vaterite replaces the aragonite. Abnormal formation of otoliths occurred in ayu from Biwa Lake (30%) and Kumamoto (45%), while the Wakayama samples had no abnormality. The microchemical analyses of the normal and abnormal otoliths indicated that some abnormal otoliths had formed before the fish were captured and transferred to the hatchery, so the possible causes of otolith abnormality in ayu are discussed.     

Enigmatic ear stones: what we know about the functional role and evolution of fish otoliths

Otoliths in bony fishes play an important role in the senses of balance and hearing. Otolith mass and shape are, among others, likely to be decisive factors influencing otolith motion and thus ear functioning. Yet our knowledge of how exactly these factors influence otolith motion is incomplete. In addition, experimental studies directly investigating the function of otoliths in the inner ear are scarce and yield partly conflicting results. Herein, we discuss questions and hypotheses on how otolith mass and shape, and the relationship between the sensory epithelium and overlying otolith, influence otolith motion. We discuss (i) the state‐of‐the‐art knowledge regarding otolith function, (ii) gaps in knowledge that remain to be filled, and (iii) future approaches that may improve our understanding of the role of otoliths in ear functioning. We further link these functional questions to the evolution of solid teleost otoliths instead of numerous tiny otoconia as found in most other vertebrates. Until now, the selective forces and/or constraints driving the evolution of solid calcareous otoliths and their diversity in shape in teleosts are largely unknown. Based on a data set on the structure of otoliths and otoconia in more than 160 species covering the main vertebrate groups, we present a hypothetical framework for teleost otolith evolution. We suggest that the advent of solid otoliths may have initially been a selectively neutral ‘by‐product’ of other key innovations during teleost evolution. The teleost‐specific genome duplication event may have paved the way for diversification in otolith shape. Otolith shapes may have evolved along with the considerable diversity of, and improvements in, auditory abilities in teleost fishes. However, phenotypic plasticity may also play an important role in the creation of different otolith types, and different portions of the otolith may show different degrees of phenotypic plasticity. Future studies should thus adopt a phylogenetic perspective and apply comparative and methodologically integrative approaches, including fossil otoliths, when investigating otoconia/otolith evolution and their function in the inner ear.     

An assessment of otoliths,dorsal spines and scales to age the long‐finned gurnard,Lepidotrigla argus,Ogilby, 1910 (Family: Triglidae)

Sagittal otoliths, dorsal spines and scales were critically assessed as structures to potentially determine the age of the long‐finned gurnard, Lepidotrigla argus. Counts were made of opaque growth increments and a readability score was assigned to each structure. Comparisons of growth increment counts were made between structures and between readings. All three structures showed some degree of readability and quantifiable growth increments, but this varied within fishes and between structures. Initial results showed that whole otoliths were more suitable to determine age estimates than dorsal spines and scales. Scales were considered unsuitable due to between reading ageing bias, variation in age estimates between structures, low precision and poor readability for this species. Dorsal spines showed evidence of loss of growth increments due to hollowing of the vascular core, which resulted in underestimation of older individuals in comparison to whole otoliths. Further analysis showed that growth increment counts from whole otoliths were lower for older individuals in comparison to sectioned otoliths. It is suggested that this is because of decreased clarity of growth increments towards the outer margin of whole otoliths in older individuals; this problem was not present with sectioned otoliths. It was concluded that sectioned otoliths were a more suitable structure from which to estimate age of L. argus than were whole otoliths, dorsal spines and/or scales.     

Morphology and ontogenetic changes in otoliths of the mesopelagic fishes Ceratoscopelus maderensis (Myctophidae), Vinciguerria attenuata and V. poweriae (Phosichthyidae) from the Strait of Messina (Mediterranean Sea)

Fish otoliths morphology is an important aid in taxonomic, phylogenetic, palaeoichthyological and dietary studies. This study compares the morphology of the otoliths of the myctophid species, Ceratoscopelus maderensis, and two phosichthyid species, Vinciguerria attenuata and V. poweriae, collected from the Strait of Messina in the central Mediterranean Sea to identify the most appropriate taxonomical characters that separate these species. Ontogenetic changes in the otoliths of the studied fishes are evident. The sagitta of the juvenile C. maderensis is the most similar to the adult otoliths, while otoliths of the juvenile V. poweriae are the least similar to the adult otoliths. A discussion on possible geographical variations in otolith's morphology is provided by comparing our results with sagitta's images of C. maderensis, V. attenuata and V. poweriae in other studies.     

Validation of age readings from otoliths of juvenile roundnose grenadier, Coryphaenoides rupestris, a deep-water macrourid fish

Validation of the ageing of deep-water fish is difficult and there are only a few instances where the rings on the otoliths have been shown to be laid down annually. Roundnose grenadier have been fished commercially in the North Atlantic since the 1960s and the adult fish have frequently been aged by counting the rings in otoliths or scales. All the ageing was done on the assumption that the rings in the otoliths or scales were annual. Between 1975 and 1992, the Scottish Association for Marine Science carried out seasonal trawling surveys in the Rockall Trough using a fine-meshed trawl, and collected otoliths from a wide size range of roundnose grenadier. An examination of the growing edge of otoliths from juvenile fish from these collections suggests that the rings in the otoliths are laid down annually. The broader, opaque zones which represent the growth phase were dominant between September and March. The thinner, hyaline zones were dominant between April and July. The apparent delay in the growth phase compared with most shallow-water species is discussed in relation to the availability of mesopelagic prey.     

Otolith size changes related with body growth,habitat depth and temperature

Synopsis Size variation in the sagittal otoliths of six species of the genus Merluccius, and five species of the genus Coelorhynchus was compared, using a digital image processing system and multivariate analysis. It is proposed that otolith growth occurs under dual regulation, overall shape is regulated genetically, and otolith size is influenced by environmental conditions. The decline of temperature with increasing habitat depth seems to be an important factor regulating the growth of otoliths in carbonate-saturated levels. The relative growth of the otoliths is usually negatively allometric.     

Otoliths of sub-adult Lake Sturgeon Acipenser fulvescens contain aragonite and vaterite calcium carbonate polymorphs

In this study we quantified the percent CaCO₃ polymorph composition in otoliths of larval and juvenile Lake Sturgeon Acipenser fulvescens via X-ray microdiffraction. Sagittal otoliths of sub-adults were primarily composed of aragonite (> 90%) while the lapilli otoliths were 100% vaterite. This is the first time the presence of aragonite in otoliths has been reported in an acipenseriform and is surprising given that the ability to form aragonite otoliths was not thought to have evolved until the separation of teleost and holostean species from other Actinopterygian fishes (e.g., sturgeon, paddlefish, gar).     

The staining of fish otoliths for age determination

Attempts were made to find methods for the staining of fish otoliths which would give results comparable to those of the Christensen burning technique. A variety of different histological stains and otoliths of different species were experimented with. Otoliths of sole, turbot, brill and scad gave best results when sectioned and stained in acidified Neutral Red, whereas those of cod, hake, whiting, plaice and grey mullet showed up the annuli better when dyed in aqueous Aniline Blue or Toluidine Blue and then sectioned. Small, translucent otoliths such as those of pelagic species may be enhanced by staining in Eosin Y.     

Mn<Superscript>2+</Superscript> concentrations in coastal fish otoliths: understanding environmental and biological influences from EPR

The Mn²⁺ concentrations in the sagittae otoliths of 12 fish families (and 19 species) that co-occur in a coastal area of southeastern Brazil (~21°S) were quantified using electron paramagnetic resonance (EPR). Inferences were made about the relationship between fish habitat and trace element incorporation. Inferences were made on the relationship between trace element concentration and otolith shape. The differences in Mn²⁺ concentrations among the species suggest that habitat (and feeding habits) might drive the incorporation of this trace element into fish otoliths, with higher values in bottom-associated fish species than in surface-associated species. In surface-associated fish species, the correlation between trace element concentrations and otolith shape was stronger than in bottom-associated species. Thus, while the Mn bioavailability in a fish’s habitat, especially from feeding resources, is a local driving influence of trace element incorporation in sagittae otoliths, species-specific requirements also have an influence. Quantitative EPR is a non-destructive technique that is very useful when the available samples cannot be damaged, like with otolith collections.     

Discriminant analysis as a tool to identify catfish (Ariidae) species of the excavated archaeological otoliths

Catfish otoliths excavated from two archaeological sites in Kuwait, Sabiyah (ca. 7000 Years Before Present) and Al-Khidr, ca. 4000 YBP, were compared with those of Kuwait’s modern catfish. Otoliths from Kuwait’s four species of catfish, Netuma bilineata, N. thalassina, Plicofollis dussumieri, and P. tenuispinis were collected after recording total length and weight. Data recorded for both ancient and modern otoliths, including annual ring (age), weight, length and four otolith radii from transverse sections, were subject to discriminant analysis to differentiate among species and develop classification functions for otoliths. Comparisons of the results from the ancient and modern otoliths showed that most of the excavated otoliths (78% from Sabiyah and 100% from Al-Khidr) belong to the two presently dominate species N. bilineata and P. tenuispinis, indicating that ichthyofauna of Kuwait Bay may not have changed much in the past 7000 years.     

Validation of a simple and well‐suited chemical cleaning method for fish otoliths

In marine biology, many research fields are based on use of fish otoliths. All the studies dealing with otoliths need as starting point a perfectly clean otolith. Dissection is difficult when working on small or highly jagged otoliths. A common problem is that during otolith preparation some fish tissue may remain stuck to it, even after a mechanical cleaning. Then, supplementary cleaning with chemicals is needed. Classical methods are known to possibly alter otolith's structure and/or composition. Here, we present a chemical cleaning method using only sodium hydroxide. We have validated the method on two different fish species, Oblada melanura and Dicentrarchus labrax. Observation and analyses of morphological measurements have confirmed significant suppression of residual tissues. We propose this method as a good alternative to previously published mono‐ or multichemical methods.     

Age determination of the Antarctic fishes Champsocephalus gunnari and Notothenia rossii marmorata from South Georgia

Summary Investigations were performed on otoliths from Champsocephalus gunnari and Notothenia rossii marmorata as part of an effort to study the population dynamics of Antarctic fishes. Examination of otoliths by scanning electron microscope (SEM) revealed internal microincrements which were similar to daily increments described for other fish species. No easily identifiable annuli could be discerned and fishes were aged by counting microincrements. Multiple regression analyses relating age to otolith morphometrics and fish size, provided a simpler ageing method. It was determined with these techniques that C. gunnari grew to 50 cm in 15 yrs and N. rossii marmorata grew to 50 cm in 6.5 yrs. The lack of larger-sized individuals for N. rossii marmorata may be indicative of recent overfishing with only the younger age groups remaining. It appears that length-at-age data determined on a yearly basis, could provide valuable scientific and management information for these species.     

Comparison of spatial variation in otolith chemistry of two fish species and relationships with water chemistry and otolith growth

Spatial variation in the chemistry (Mg, Mn, Sr and Ba) of recently deposited otolith material (last 20–30 days of life) was compared between two demersal fish species; snapper Pagrus auratus (Sparidae) and sand flathead Platycephalus bassensis (Platycephalidae), that were collected simultaneously at 12 sites across three bays in Victoria, south-eastern Australia. Otolith chemistry was also compared with ambient water chemistry and among three sampling positions adjacent to the proximal otolith margin. For both species, variation in otolith chemistry among bays was significant for Ba, Mn and Sr; however, differences among bays were only similar between species for Ba and Mn. Only Ba showed significant variation at the site level. Across the 12 sites, mean otolith Ba levels were significantly positively correlated between species. Further, although incorporation rates differed, mean ambient Ba levels for both species were positively correlated with ambient Ba levels. Spatial variation in multi-element otolith chemistry was also broadly similar between species and with multi-element water chemistry. Partition coefficients clearly indicated species-specific incorporation of elements into otoliths. Mg and Mn were consistently higher in snapper than sand flathead otoliths (mean ±s .d ., Mg snapper 22·1 ± 3·8 and sand flathead 9·9 ± 1·5 μg g⁻¹, Mn snapper 4·4 ± 2·6 and sand flathead 0·5 ± 0·3 μg g⁻¹), Sr was generally higher in sand flathead otoliths (sand flathead 1570 ± 235 and snapper 1346 ± 104 μg g⁻¹) and Ba was generally higher in snapper otoliths (snapper 12·1 ± 12·8 and sand flathead 1·8 ± 1·4 μg g⁻¹). For both species, Mg and Mn were higher in the faster accreting regions of the otolith margin, Sr was lower in the slower accreting region and Ba showed negligible variation among the three sampling regions. This pattern was consistent with the higher Mg and Mn, and generally lower Sr observed in the faster accreting snapper otoliths. It is hypothesized that the differences between species in the incorporation of these elements may be at least partly related to differences in metabolic and otolith accretion rate. Although rates of elemental incorporation into otoliths appear species specific, for elements such as Ba where incorporation appears consistently related to ambient concentrations, spatial variation in otolith chemistry should show similarity among co-occurring species.