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Seasonal detection rates of river otters (Lontra canadensis) using bridge-site and random-site surveys

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Randomization of survey sites is generally desired because of its unbiased approach, but is often abandoned because of logistical constraints. This is true for river otters (Lontra canadensis (Schreber, 1777)), with bridges commonly determining survey locations. We conducted seasonal sign surveys for river otters on two rivers in southern Missouri, USA, using randomized survey points and fixed bridge-crossing points in 2001-2003. Otter sign was more likely to be detected at randomized sites than at bridge sites in summer (P < 0.0001), with sign being detected on 68% of visits to random sites (n = 348) and on 40% of visits to bridge sites (n = 60). Scat abundance was higher (P = 0.0001) at random sites (8.82 ± 0.6, mean ± SE) than at bridge sites (3.96 ± 1.0) during the summer. Similar but nonsignificant trends were found during the winter. Detection probabilities were significantly higher at random sites than at bridge sites in both seasons. Our results indicate that surveys of bridge sites for river otters may yield inaccurate results for distribution and relative abundance, particularly if conducted during the summer.

On recherche généralement à établir une répartition aléatoire des sites d’inventaire à cause de son caractère non biaisé, mais les contraintes logistiques nécessitent souvent son abandon. C’est le cas des loutres de rivière (Lontra canadensis (Schreber, 1777)) chez lesquelles les ponts déterminent souvent les sites d’inventaire. Nous avons mené des inventaires saisonniers des traces de loutres de rivière dans deux rivières du sud du Missouri, É.-U., en utilisant des sites aléatoires d’inventaire et des sites fixes aux croisements des ponts en 2001-2003. Les signes de loutres sont plus susceptibles d’être décelés aux sites aléatoires qu’aux sites près des ponts en été (P < 0,0001), ces signes étant observés lors de 68 % des visites aux sites aléatoires (n = 348) et de 40 % des visites aux sites près des ponts (n = 60). L’abondance des fèces est plus élevée (P = 0,0001) aux sites aléatoires (8,82 ± 0,6, moyenne ± erreur type) qu’aux sites près des ponts (3,96 ± 1,0) durant l’été. Il existe les mêmes tendances, bien que non significatives, en hiver. Les probabilités de détection sont significativement plus élevées aux sites aléatoires qu’aux sites près des ponts aux deux saisons. Nos résultats indiquent que les inventaires aux sites près des ponts peuvent produire des résultats inexacts sur la répartition et l’abondance relative, particulièrement s’ils sont faits en été.

Document Type: Research Article

Publication date: November 1, 2009

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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