Bluefish
(also known as Albacora, Anchova, Anchoveta, Anchovinha, Piquitinga, Anxova, Enchova, and Enchoveta-baeta in Brasil; Snapping mackerel, Skipmackerel, Skipjack, Horsemackerel, and Fatback in Guyana; Tailor run and Anjova in Cuba; Razorbacks, Jumbos, and Choppers in Australia)

Pomatomus saltatrix (Linnaeus, 1766)
(previously identified as Cheilodipterus heptacanthus (Lacepède, 1801), Cheilodipterus saltatrix (Linnaeus, 1766),  Chromis epicurorum (Gronow, 1854),  Gasterosteus saltatrix (Linnaeus, 1766), Gonenion serra (Rafinesque, 1810), Lopharis mediterraneus (Rafinesque, 1810), Perca lophar (Forsskål, 1775), Pomatomix saltatrix (Linnaeus, 1766), Pomatomus pedica (Whitley, 1931), Pomatomus saltator (Linnaeus, 1766), Pomatomus skib (Lacepède, 1802), Scomer sypterus (Pallas, 1814), Sparactodon nalnal (Rochebrune, 1880), Sypterus pallasii (Eichwald, 1831), Temnodon conidens (Castelnau, 1861), Temnodon saltator (Linnaeus, 1766), and Temnodon tubulus (Saville-Kent, 1893))

Bluefish otoliths are extremely fragile and often break during the removal process. Once extracted, to safeguard against breakage during transport, otoliths are housed in standard 1.9 ml micro-centrifuge vials. Each vial contains a specimen’s pair of otoliths and is stored in a 6cm x 10.5cm coin envelope. The envelope is labeled with information that is applicable to the specimen such as date, weight, sex, and species.

When dealing with ageing structures it is important to make use of standardized terminology when describing the morphology of the structure.

These figures illustrate otolith morphology terminology commonly used when describing the otolith and its orientation within the head.

Isomet saw

Thermolyne furnace

CQFE uses a "bake and thin-section" technique to process bluefish otoliths for age determination. Otolith preparation begins by randomly selecting either the right or left otolith and making a transverse cut tusing two 7.6 cm x 0.15 mm diamond wafering blades seperated by a 0.5mm spacer on a Buehler Isomet low speed saw.

The cutting arm of the Isomet saw has been modified to hold a block of modeler’s clay or "plastercine" that locks the otolith in place during the cutting process. If necessary, the cut surface of the otolith containing the focus is trimmed perpendicular to long axis of the otolith to ensure the cut surface will sit horizontal on a glass slide. After trimming the otolith is baked in a Thermolyne 1400 furnace at 400° Celsius for approximately 1.5 to 2.0 minutes, or until the otolith turns a light caramel color. The otolith section is then affixed to a 1.0mm diameter microscope slide with "Loctite 349". Loctite is an optically clear ultraviolet adhesive that requires a multiband UV-254/366NM catalysis to initiate curing. Depending on atmospheric conditions such as humidity and temperature, sufficient curing usually takes approximately 24 hours.

Ecomet grinder

Reading

The otolith preparation method described above enhances the winter growth (opaque) bands, which by convention are counted as annual marks or annuli. Even with this bake and thin-section process, interpretation of the growth zones from the otoliths of young bluefish is difficult. Rapid growth within the first few years of life prevents sharp delineation between opaque and translucent zones. Typically the first year’s annulus can be found by locating the focus of the otolith, which is a visually distinct dark oblong region found in the center of the otolith. If the otolith has been accurately sectioned the sulcal groove will come to a sharp point within the middle of the focus . The first year’s annulus has the highest visibility proximal to the focus along the edge of the sulcal groove. Once located, follow the first year’s annulus outward from the sulcal groove towards the dorsal perimeter of the otolith. Often, but not always, the first year is associated with a very distinct crenellation on the dorsal surface and a prominent protrusion on the ventral surface.

 The figure demonstrates the location of the first year’s annulus and associated landmarks of a one-year-old bluefish. Unfortunately both these landmarks have a tendency to become less prominent as the fish becomes older. Figure 8 demonstrates how the dorsal crenellation and the ventral protrusion dissipate with increased age. Reading commences from a point on the dorsal margin of the sulcal groove and continues in a proximal direction away from the focus. Annuli on bluefish are most evident in this region and are represented as opaque lines that end abruptly when they reach the margin of the sulcal groove 

Scales

Carver press

Body areas where the most readable scales are located are species specific. To determine which area contain scales best for ageing bluefish, scales are collected from five different locations on the museum side of the specimens. A comparison study will reveal if different locations lead to different age interpretations. This study is currently in progress and as of yet no conclusions can be drawn.

Due to the extreme variation in the size and shape of bluefish scales, only those that had even margins and uniform size were selected. As well, selected scales included those that revealed no evidence of scale regeneration or absorption. Six to ten of these preferred scales were selected and prepared as impressions. Impressions were made on extruded clear 0.20 gauge acetate sheets (25mm x 75mm) with a Carver Laboratory Heated Press (model "C"). The scales were pressed with the following settings:

Pressure: 15 000 to 20 000 psi
Temperature: 100 - 130° C
Time: approximately 7.0 to 10.0 minutes

This figure illustrates the terminology commonly used when describing the scale morphology and orientation on the body.

Age determination of bluefish scales is conducted with a standard Bell and Howell microfiche reader, with magnification of 10 to 20X.

Reading

Annuli on bluefish scales are primarily identified by the presence of crossing over of the circuli. Crossing over is most evident on the lateral margins near the posterior\anterior interface of the scale. Here compressed circuli "cross over" the deposited circuli of the previous year’s growth. Typically the annulus will protrude partially into the ctenii of the posterior field, but not always.

 Furthermore, the annuli will appear to split at the posterior/ anterior interface . This is typically occurs in the first three to four years of growth. Following the annulus up into the anterior field of the scale reveals a pattern of discontinuous and suddenly breaking, segmented circuli. This region is also distinguished by the presence of concentric white lines, which are associated with the disruption of the circuli. This pattern is most apparent in the lateral margins of the anterior field of the scale. Interpretation of the annuli in the anterior field becomes somewhat more difficult due to lack of resolution.

This figure illustrates the location and reading plane of a five year old bluefish.

Vertebrae

The vertebrae of bluefish have been used in the past for age determination (Johnson 1979; Barger 1990). The Age and Growth Laboratory has found bluefish vertebrae useable as an alternative structure for age determination with the understanding that the structure will tend to underestimate fish age. Typically the first year is difficult to locate, as are the annuli on the outer margin of the structure, especially if any connective tissue remains attached. The convention of age determination by way of vertebrae is numeration of the concentric ridges and the depressions that follow. Although vertebrae do have light and dark zones that appear to be annuli, this is not the case. Vertebrae of bluefish have concentric ridges and depression that together constitute an annulus.

This figure illustrates the terminology used to describe the morphology and orientation of the vertebrae.

Preparation of vertebrae requires that all tissues and cartilage be removed. Boiling the structure for several minutes allows for most of the tissues, gelatinous materials, and oils to be removed from the vertebrae. A short bath in 5% hypochloride (bleach) whitens the structure and allows for increased resolution. The vertebrae are then stained in a 0.01% solution of crystal violet until the structure has reached a uniform blue color.

A second preparation method that has been investigated is clearing the vertebrae in "linseed oil". The clearing process lightens the structure and allows for increased resolution. However, light and dark zones become too apparent, to the point that it distracts from the visibility of the concentric ridges and depressions. The reader must scrutinize the structure to ensure that the ridges and depressions are counted and not the light and dark bands.

After drying, the vertebrae are cut with an Isomet saw through the midline of the neural arch.

Microscopy

Age determination of bluefish vertebra is done with a dissecting microscope at between 8 and 100X magnification. Reflected light is used with a black background.

Reading

First, the vertebra is positioned so the field of view contains the entire cut surface. Beginning with the centrum, the surface is read towards the outer margins of the vertebra. Again, it is the combination of ridges and depression that constitute the annulus, and not the combination of light and dark zones. The first year is located near the core of the centrum, which can be difficult to locate. The cut surfaces on both sides of the vertebrae are scanned for ridges and depressions. As reading continues towards the outer margin, the reader visually sweeps back and forth so that the ridges and depressions on the cut surfaces flow into the ridges and depression within the centrum of the vertebrae. Particular attention should be paid to the outer lip of the vertebra where annuli become very compact and may be difficult to identify. If necessary a light coating of mineral oil is used to enhance the resolution. As with any ageing structure the reader must manipulate and reorientate the structure such so the best field of view and light contact is acquired.

Pectoral Fin Rays

The pectoral fin ray of bluefish can be used as an alternative structure for age determination with the understanding that the structure will generally underestimate fish age. Two problematic regions are the first year’s growth, and the slowing of growth near the outer margins of older, or mature fish.

The first year’s annulus is deposited near the central core of the fin ray. This highly vascularized area makes identification of the first year extremely difficult. Furthermore, rapid growth of young bluefish results in an enlarged and diffused translucent zone, or annulus, that reduces the resolution of the first year. Older or mature fish experience crowding of the annuli near the outer margins of the fin rays. These translucent zones are extremely fine, and difficult to segregate from the translucent summer zone.

This figure illustrates morphological terminology and orientation of the fin ray. Fin Rays are processed by first allowing the structure to dry with rays completely fanned open. Incidental muscle and dermis can be removed at this point, however, care must be taken not to remove too much of the dermis since it maintains fin ray integrity and keeps the individual rays together during the sectioning process. The entire fin ray or selected rays, depending on the size of the structure, are sectioned. Typically small fin rays are imbedded in an epoxy resin that gives the structure support during the sectioning process. However, the larger more robust fin rays are simply pressed into a plasticine block and sectioned with a cutting saw. The fin rays are sectioned perpendicular to the long axis of the fin. At times this can be difficult due to the natural curve of the rays within the fin complex. The initial cut is made as close to the articulation site as possible, followed by multiple cuts that produce serial sections from the ray bases and working towards the tip of the fin. This ensures that the first year's annulus will be contained within the section.

Typically the first year's annulus disappears as the sections progress towards the tip of the fin ray. The thickness of the bluefish pectoral sections depends on the size of the specimen. Small bluefish, 500mm (TL) and under, are sectioned between 0.5 and .75mm. The smaller the structure the more translucent it becomes. If the sections on smaller fish are too thin, no visible annuli will be detected. Bluefish larger then 500mm (TL) are sectioned between 0.25 and 0.5mm thick. It is recommended that only several fish are processed at once, and those sections scrutinized for quality control.

Sections are then organized onto microscope slides in the order that they were sectioned, which allows the age reader to locate and follow the first year's annulus through its presence in the ray. Fin ray sections are then affixed to the microscope slide with a clear mounting medium (flo-texx). This also serves to maintain a high gloss finish and eliminates any cut marks left behind by the cut blade.

Microscopy

Age determination of bluefish fin rays is achieved with a dissecting microscope with a magnification range of 8 to 100X. Reflected light is used on a black background.

Reading

Fin rays are composed of biserial filaments; the larger of the two elements is examined for age determination on bluefish. Typically those cores that are closest to the articulation site are large and encompass much of the filament. This core area becomes smaller as the serial sections move outwards to the tip of the fin ray. The larger element of a ray that has the best resolution and clarity is sought for ageing. The first year’s annulus can be found directly around the core of the fin ray. Once the first year has been located the best reading plane out towards the edge of the structure is followed. Once an age has been established for that particular element, the same element on the next serial section is aged, continuing for all sections that are readable.

Age your bluefish

Age by Length

Age by Weight