Malcolm P. Francis

Extinction risk and conservation of the world’s sharks and rays

The rapid expansion of human activities threatens ocean-wide biodiversity. Numerous marine animal populations have declined, yet it remains unclear whether these trends are symptomatic of a chronic accumulation of global marine extinction risk. We present the first systematic analysis of threat for a globally distributed lineage of 1,041 chondrichthyan fishes—sharks, rays, and chimaeras. We estimate that one-quarter are threatened according to IUCN Red List criteria due to overfishing (targeted and incidental). Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays. Overall chondrichthyan extinction risk is substantially higher than for most other vertebrates, and only one-third of species are considered safe. Population depletion has occurred throughout the world’s ice-free waters, but is particularly prevalent in the Indo-Pacific Biodiversity Triangle and Mediterranean Sea. Improved management of fisheries and trade is urgently needed to avoid extinctions and promote population recovery. DOI: http://dx.doi.org/10.7554/eLife.00590.001

Sex-biased dispersal of great white sharks

In some respects, these sharks behave more like whales and dolphins than other fish.

Morphometry of the stone loach, Barbatula barbatula : do mensural characters reflect the species' life history thresholds?

Growth variability in 23 mensural characters was examined in 387 specimens of stone loach, Barbatula barbatula, from England. The standard length (SL) of the specimens ranged from 15.3 to 115.4 mm. We tested the hypothesis that body proportions change abruptly, rather than gradually, at certain intervals of ontogeny by fitting linear, quadratic and split linear curves to plots of each variable against SL. Based on patterns of allometric growth, two groups and two subgroups of mensural characters have been found. Three characters were best explained by a linear regression, indicating isometric growth. Eight characters were best explained by a quadratic curve, indicating gradual allometry. The remaining 12 characters were best explained by a split regression, indicating mainly isometric growth with abrupt allometry occurring at a specific SL (breakpoint). The first shift in morphometric values (a transformation of the head; breakpoints in three characters) occurred at 26-35 mm SL, the second (a change in fin shape and size as well as body form; breakpoints in six characters) at 36-47 mm SL. The coincidence of shifts in body morphology with those in microhabitat use (between the respective size classes) suggests that thresholds (though not as sudden as those between embryo and early larva steps) do occur during this interval of stone loach life history. We suggest that the larva period ends with the completion of the first shift in relative growth (i.e. not later than at 35 mm SL, depending on individual variability), and that the second shift in morphometric values reflects a threshold between the first and the second step of juvenile period. The importance of changes in external morphology decreased as the fish grew bigger and older.

Age under-estimation in New Zealand porbeagle sharks (Lamna nasus): is there an upper limit to ages that can be determined from shark vertebrae?

Annual deposition of growth bands in vertebrae has been validated for many shark species, and is now widely regarded as the norm. However, vertebrae are part of the sharks axial skeleton, and band deposition may stop in old sharks when somatic growth ceases. We aged vertebral sections from New Zealand porbeagle sharks (Lamna nasus) under reflected white light and using X-radiographs. Bomb radiocarbon assays supported vertebral age estimates up to ∼20 years, but not at older ages. The results suggest that older porbeagles were under-aged by as much as 50% from vertebral band counts, presumably because band width declined to a point where it became unresolvable. This has important implications for growth studies on other long-lived sharks. Estimated ages at sexual maturity were 8-11 years for males and 15-18 years for females, and longevity may be ∼65 years. New Zealand and North Atlantic porbeagles differ in these parameters, and in length at maturity and maximum length, suggesting genetic isolation of the two populations.

Age, growth, maturity, longevity and natural mortality of the shortfin mako shark (Isurus oxyrinchus) in New Zealand waters

Shortfin mako sharks were aged by counting growth bands in sectioned vertebrae (n = 256), and assuming annual band-pair deposition. No systematic ageing bias was present and count precision was high. 0+ juveniles were identified from length-frequency plots and assigned ages based on a theoretical birth date of 1 October and their date of capture. A Schnute generalised growth model fitted to the combined vertebral and 0+ data described the growth patterns best. Shortfin makos grow very rapidly initially, increasing by ∼39 cm fork length in their first year. Thereafter, males and females grow at similar but slower rates until about age 7 years, after which the relative growth of males declines. Longevity estimates were 29 and 28 years for males and females respectively. Natural mortality (M) is probably in the range of 0.10-0.15. Median ages at maturity were 7-9 years for males and 19-21 years for females. Comparisons of growth curves reported here and elsewhere suggest no regional differences in growth rates. The shortfin mako is a late-maturing species with moderate longevity and low natural mortality. With these life history characteristics and an unknown stock size and structure worldwide, management should be of a precautionary nature.

Geographic distribution of marine reef fishes in the New Zealand region

Abstract The geographic distributions of 375 reef and reef‐associated fishes are reported for 16 regions ranging from Norfolk and Kermadec Islands in the north to Macquarie Island in the south. Species diversity was greatest at Norfolk Island (228 species) and lowest at Macquarie Island (6 species). Diversity declined linearly with increasing latitude. Most species were either widespread or had very restricted distributions. Widespread species generally ranged from Three Kings Islands to Stewart Island. The most widespread species occurred in 14 of the 16 regions. Species with restricted distributions were mainly tropical or subtropical species that occurred at one or more of Norfolk Island, Kermadec Islands, and North‐East North Island. Principal Components Analysis (PCA) identified eight groups of regions with similar species compositions. These eight groups reflected the latitudinal variation in sea surface temperature. Seven species distributional groups were recognised. Tropical (167 species) and ant...

Morphometric minefields—towards a measurement standard for chondrichthyan fishes

Size measurements are crucial for studies on the growth, maturation, maximum size, and population structure of cartilaginous fishes. However, researchers use a variety of measurement techniques even when working on the same species. Accurate comparison of results among studies is only possible if the measurement technique used is adequately defined and, if different techniques are used, a conversion equation can be derived. These conditions have not always been met, leading to invalid comparisons and incorrect conclusions. This paper reviews methods used for measuring chondrichthyans, and summarises the variety of constraints that influence the choice of a measurement technique. Estimates of the variability present in some measurement techniques are derived for shortfin mako shark, Isurus oxyrinchus, porbeagle shark, Lamna nasus, blue shark, Prionace glauca, Antarctic thorny skate, Amblyraja georgiana, and Pacific electric ray, Torpedo californica. Total length measured with the tail in the natural position (sharks) and disc widths (batoids) have higher variability than other methods, and are not recommended. Instead, the longest longitudinal axis should be measured where possible and practical; i.e., flexed total length for sharks, total length for batoids (excluding suborder Myliobatoidei), pelvic length for batoids of the suborder Myliobatoidei, and chimaera length (snout to posterior end of supracaudal fin) for chimaeroids (except for Callorhinchus, for which fork length should be measured from the anterior edge of the snout protuberance). Straight-line measurements are preferred to measurements over the curve of the body. Importantly, measurement methods must be clearly defined, giving information on the anterior reference point, the posterior reference point, and how the measurement was made between these two. Measurements using at least two different methods are recommended on at least a subsample of the fish in order to develop conversion regression relationships.

New Zealand Demersal Fish Assemblages

Demersal fish assemblages in the New Zealand Exclusive Economic Zone were identified using presence–absence data from 19 215 bottom trawl tows made over a 37-year period. The dataset spanned latitudes 34–54°S and depths of 4–1500 m. A total of 123 taxa occurred in more than 1% of the tows (121 fish and 2 squid). Multivariate ordination and classification (correspondence analysis and Wards cluster analysis) identified four primary species assemblages that were associated with the inner continental shelf, mid–outer continental shelf and shelf edge, upper continental slope and mid continental slope. The most frequently occurring species (> 40% of tows) in each assemblage were (in descending order): inshore – Chelidonichthys kumu, Pagrus auratus and Zeus faber; shelf – Nototodarus spp., Squalus acanthias and Thyrsites atun; upper slope – Macruronus novaezelandiae, Lepidorhynchus denticulatus, Genypterus blacodes and Hydrolagus sp.; mid slope – Hoplostethus atlanticus, Etmopterus baxteri, Halargyreus johnsonii, Coryphaenoides subserrulatus, Deania calcea, Coryphaenoides serrulatus, Pseudocyttus maculatus, Mora moro, Diastobranchus capensis and Centroscymnus crepidater. Further species associations were also identified within each primary assemblage. Canonical correspondence analysis revealed that most of the explainable variation in species composition was associated with depth and latitude; longitude and season explained little extra variance. The usefulness of our results is limited by the use of presence–absence rather than abundance data, and by the uneven spatial distribution of trawl tows. However, the present study provides a large-scale framework within which to interpret the results of studies using abundance data over smaller spatial and temporal scales.

Pelagic shark bycatch in the New Zealand tuna longline fishery

Tuna longline effort declined from 23—26 million hooks per year in 1979mdash;82 to 2mdash;4 million hooks per year in 1995mdash;98. Scientific observer coverage averaged 7.5%since 1988mdash;89, but increased in 1992mdash;93 (mean 23%). Observed catch per unit effort (CPUE) and the numbers of hooks set were used to estimate shark catches. Between 1988mdash;89 and 1997mdash;98,about 450 000 blue sharks (Prionace glauca), 65 000 porbeagles (Lamna nasus ) and 25 000 shortfin makos (Isurus oxyrinchus) were caught. In 1997mdash;98,about 45 000 blue sharks, 4000 porbeagles and 3000 makos were caught. Corresponding weight estimates were 1400 t, 150 t and 200 t. CPUE varied between foreign and domestic vessels, between north and south regions and among years, but there were no consistent temporal trends. Most males and females were immature, and most sharks were alive when recovered. Most sharks were processed, but usually only the fins were retained. The New Zealand tuna longline fishery is probably not seriously affecting pelagic shark stocks, but adequate assessment is not currently feasible. Accurate monitoring of Pacific Ocean catches is an important first step towards ensuring sustainability of their populations.

Age, growth, and sexual maturity of two New Zealand endemic skates, Dipturus nasutus and D. innominatus

Abstract Rough and smooth skates (Dipturus nasutus (Banks 1841) and/), innominatus (Garrick & Paul 1974)) were aged by counting growth bands on X‐rays of thick sections of vertebral centra. Band counts were imprecise, but there was no between‐reader bias. Age estimates were not validated. The oldest rough skate was 9 years old, but few were more than 6 years old. Females may live longer than males. The combined sexes von Bertalanffy growth curve was Lr = 91.3 (1 ‐ e−0.16[t + 1.20]). Half the males matured by c. 52 cm pelvic length (PL) and 4 years, and females by 59 cm PL and 6 years. The oldest smooth skate in the sample was 24 years, but longevity probably exceeds that. Females appear to live longer than males. The combined sexes von Bertalanffy growth curve was: Lt = 150.5 (1 ‐ e−0.095[t + 1.06]). Half the males matured by c. 93 cm PL and 8 years, and females by 112 cm PL and 13 years. Smooth skate are late maturing and long‐lived relative to other skates, whereas rough skate are early maturing with a moderate life span.