NOTE

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

D. J. Booth

Received: 10 June 2013 / Accepted: 12 November 2013 / Published online: 23 November 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract 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 for-

mation. However, otolith increment widths were poor

proxies for actual growth in length [linear regression

r2 = 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 incre-

ments, suggesting the long-term back calculations of oto-

lith growth characteristics of reef fish populations should

be interpreted with caution.

Keywords Age validation � Coral reef damselfish �Growth � Otolith back calculation �Otolith increments � Pomacentrus moluccensis

Introduction

A key goal of ecological research is to understand how

species vary in their persistence. Performance measures

include survivorship and growth, and studies often assess

how these demographic parameters vary temporally and

spatially. For marine fishes, a structure often used to

hindcast survival and growth is the otolith or earstone,

which accretes rings that have been calibrated both as

annual (Fowler and Short 1998) and daily (Panella 1971).

Validation of these rings as annual can be done in the

laboratory by staining otoliths and holding fish for over a

year, or in the field using a marginal otolith increment

approach (Fowler and Short 1998) or by comparing with

known age of a tagged fish. The latter is the most reliable

but is rarely done since it requires a long-term (multiyear)

field monitoring program. Shorter-term field annual ring

validation consisted of marking the fish with a vital stain

such as tetracycline and recapturing the fish after 1–2 years

(e.g., Fowler and Short 1998) and thus cannot be applied to

the entire life span of longer lived fish.

More recently, the size of increments between otolith

rings has been used as a proxy for somatic growth,

allowing individual and population trajectories of growth

to be assessed (Campana 1990). Field time is also mini-

mized compared to mark-recapture, and a selection of

cohorts can be sampled at each site by sampling otoliths at

one point in time. However, assumptions of daily and

annual formation of rings and use of increments as growth

proxies have to be validated (Campana 1990). Regarding

the use of otolith increments as proxies for growth stanzas,

studies have shown that increments can decouple for

somatic growth, for short or longer periods (e.g., Wright

et al. 1990). While a number of statistical assumptions

must go into choice of back calculation model, validation

Communicated by Biology Editor Dr. Glenn Almany

D. J. Booth (&)

School of the Environment, University of Technology, Sydney,

PO Box 123, Broadway, NSW 2007, Australia

e-mail: david.booth@uts.edu.au

123

Coral Reefs (2014) 33:255–258

DOI 10.1007/s00338-013-1105-2

of how otolith growth relates to somatic growth is often

lacking (Vigliola et al. 2000).

Here, based on a long-term study on the Great Barrier

Reef of a common coral reef damselfish (Pomacentrus

moluccensis), I ‘‘ground-truth’’ otolith-based estimates of

longevity and growth against actual data from tagged fish.

Materials and methods

I tagged fish at two sites in and adjacent to One Tree Island

lagoon, in the southern Great Barrier Reef (23�300S,

152�060E) as part of a long-term study of spatial and

temporal demographics. Cohorts of newly recruited fish

(size = 15 mm TL on average; 25–35 days post-hatching

after planktonic larval dispersal) were tagged with Visual

Implant Flourescent Elastomer paint and individuals mea-

sured (total length, TL mm) and followed annually. As a

part of the study, commenced in 2000, I recaptured fish and

recorded body length annually (mm TL). Six tagged fish

were recaptured and killed in 2006 (2 from one site and 4

from another) and measured in situ. Fish were collected

using SCUBA and with clove oil anaesthetic, and frozen

for transport to the University of Technology, Sydney,

where otoliths (sagittae) were removed by dissection and

stored dry until ready for processing. Otoliths were ground

to the primordium using 3 lm lapping film; wet polished

with a PMT 08A-10 multiflex 8’’ diameter polishing cloth

using 0.02–0.06 lm colloidal silica on a Kent 3—auto-

matic polishing unit; and then etched using a 5 % solution

of EDTA (ethylenedinitrilo tetra acetic acid). Prepared

otoliths were viewed at 40–809 on an Olympus BH2-

RFCA compound microscope, images stored, and putative

annual increments counted. The number of growth incre-

ments was counted along the most visible axis of the otolith

three times, with the mean taken as the age of the fish. If

counts differed by more than 5 %, otoliths were reexam-

ined and if subsequent counts again varied by over 5 %,

otoliths were rejected (sensu Campana and Neilson 1982).

Increment widths (primordium to putative Year One

Increment, Year One to Year Two Increment, etc.) were

measured along a consistent axis in microns, from the

digital images using Image J software.

Results and discussion

The same six fish that had been externally tagged soon after

settlement were monitored yearly for body size and col-

lected at year six for otolith age determination and mea-

surement of growth increments. First, putative annual age

rings on otoliths of P. moluccensis were validated, i.e., for

all six fish that were recaptured after 6 years, there were 6

opaque zones. Second, for each fish, the relationship

between annual actual somatic growth (mm TL) and otolith

increment width was plotted. If otolith increment width

was a perfect proxy for somatic growth, an r2 approaching

1 was expected. A pooled r2 of 0.61 indicated a poor

predictive value overall of otolith increment width for

somatic growth (Fig. 1). For each fish, r2 varied from 0.44

to 0.90. In addition, Fig. 1 shows that otolith growth was

positive even in years where fish exhibited no or negative

growth in length (see also Caldow and Wellington 2003).

Figure 2 shows that how annual variation in somatic

growth and otolith increment width changes across 6 years

for each of the 6 tagged fish. Clearly, increment width

patterns are poor proxies of actual somatic growth (length)

patterns for all fish. Campana (1990) corrected biases

associated with proportionally larger otoliths for slower-

growing individuals, but clearly, such corrections would

not rectify the large mismatch between increment widths

and somatic growth demonstrated here. Wright et al.

(1990) demonstrated otolith/somatic growth decoupling in

salmon was likely due to metabolic differences among

individuals, with slow growers having a relatively higher

increment width for a given somatic growth, and this may

be occurring in the present study where otolith annual

increments of at least 5 lm were seen even in fish that did

not grow (Fig. 1). However, it still does not explain the

poor relationship within an individual fish.

Using the long-term tag monitoring, annual formation of

otolith rings of a common coral reef damselfish, P. mo-

luccensis, was validated at one location. This confirms one-

year tetracycline validations for this species (Fowler and

Doherty 1992) and shows ontogenetic and spatial consis-

tency. However, the use of otolith increment width as a proxy

for somatic growth was not accurate. Over 60 % error can be

expected for this species where otolith increment width is

Fig. 1 Relationships between actual annual growth (mm year-1) and

otolith annual increment width (lm) for 6 tagged P. moluccensis

256 Coral Reefs (2014) 33:255–258

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used to indicate annual growth patterns. In addition, several

studies have noted more difficulty in interpreting otolith

growth rings from fish in tropical versus temperate marine

environments (e.g., for damselfishes: Caldow and Welling-

ton 2003, and see Morales-Nin and Panfili 2005 for a

review). Therefore, commonly used otolith back calculation

estimates of fish growth on coral reefs should be used with

caution at both the individual and population level. Given the

importance of demographic models in conservation and

management of reef fishes, more emphasis should be given to

understand the role and biases associated with common uses

for otolith information in this context.

Acknowledgments I would like to thank Gigi Beretta, Will Figueira,

and Ralph Alquezar for their help in monitoring and capturing fish and

the staff of One Tree Island Research Station for their support. Many

thanks to Kerryn Parkinson for her expert assistance with otolith

preparations and to Ash Fowler for comments on the draft manuscript.

This is contribution 119 of the Sydney Institute of Marine Sciences.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2 Annual change in growth of six tagged P. moluccensis (mm year-1, dotted line) versus otolith annual increment width (lm, solid line)

Coral Reefs (2014) 33:255–258 257

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References

Caldow C, Wellington GM (2003) Patterns of annual increment

formation in otoliths of pomacentrids in the tropical western

Atlantic: implications for population age-structure examination.

Mar Ecol Prog Ser 265:185–195

Campana SE (1990) How reliable are growth back-calculations based

on otoliths? Can J Fish Aquat Sci 47:2219–2227

Campana SE, Neilson JD (1982) Daily growth increments in otoliths

of Starry Flounder (Platichthys stellatus) and the influence of

some environmental variables in their production. Can J Fish

Aquat Sci 39:937–942

Fowler AJ, Doherty PJ (1992) Validation of annual growth

increments in the otoliths of two species of damselfish from

the Southern Great Barrier Reef. Aust J Mar Freshw Res

43:1057–1068

Fowler AJ, Short DA (1998) Validation of age determination from

otoliths of the King George whiting Sillaginodes punctata

(Perciformes). Mar Biol 130:577–587

Morales-Nin B, Panfili J (2005) Seasonality on the deep-sea and

tropics revisited: what can otoliths tell us? Mar Freshw Res

56:585–598

Panella G (1971) Fish otoliths: Daily growth layers and periodical

patterns. Science 173:1124–1126

Vigliola L, Harmelin-Vivien M, Meekan MG (2000) Comparison of

techniques of back-calculation of growth and settlement marks

from the otoliths of three species of Diplodus from the

Mediterranean Sea. Can J Fish Aquat Sci 57:1291–1299

Wright PJ, Metcalfe NB, Thorpe JE (1990) Otolith and somatic

growth rates in Atlantic salmon parr, Salmo salar L: evidence

against coupling. J Fish Biol 36:241–249

258 Coral Reefs (2014) 33:255–258

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